Hmm....
First, to respond to the original post: in my opinion, there is no such thing as a mild psychedelic. 4-HO-MET, AL-LAD, and 2C-C have all given me some of the deepest and more powerful psychedelic experiences I've ever had, even though they were at the same time less confusing than most LSD or mushroom trips for me. If you want mild psychedelia as a starter, just take low dosages of something first and work your way up, but on that note, I'll also add that I like some others here am also of the opinion that actually starting with a reasonably intense trip is likely to be a better introduction to psychedelics than working your way up slowly as long as you're properly prepared for it, but you should do whatever makes you feel most comfortable when you take it. While there are plenty of good options to start with, I would very much have to agree based on your requirements that 4-HO-MiPT may be an excellent choice. First of all, the majority of worthwhile lysergamides and phenethylamines are out based on taking too long, often at least as if not longer than LSD, so if you're avoiding LSD already for that reason that clearly won't do. The handful of shorter-acting phenethylamines already mentioned plenty such as 2C-B and 2C-C are also very fun and worthwhile and I generally wouldn't have much of a problem with recommending them to a first-timer, however I have so far found in my travels and those I have encountered (though of course others may have a totally different story still) that phenethylamine psychedelics in general are more likely to lead to uncomfortable bodily sensations while tripping than tryptamine psychedelics are, for the most part, and this is something I see as bad because it could add a negative edge to your first time both in terms of simple enjoyment and possibly in terms of creating a more negative headspace overall even with the often mentally lighter trips like these short-lived phenethylamines, though for the record I still highly recommend them in general as a bad body load is something you can look beyond with experience if you really want to get something out of a psychedelic.
As for tryptamines, while I do agree that base and 5-substituted tryptamines are more easygoing in a lot of ways than 4-substituted tryptamines, first of all they both have a much greater potential to produce physical side effects in me, some of the worst body loads in the whole psychedelic experience for the 5-substituted tryptamines and potentially very powerful purges for the base tryptamines, and I also find their trips to be not that mental but still quite spectacular and overwhelming when really explored in a way that I feel requires some level of psychedelic experience to properly make use of. 4-Substituted tryptamines may have a bit more of a mindfuck than most things, but they're usually very clean on the body, easy to use, and typically not that overwhelming relative to other extremely powerful psychedelis even though they might feel like it at the time, and plus they last for a shorter amount of time as requested but also not too short. Actually contained within the realm of the 4-substituted tryptamines then, there are still plenty of great options to choose from, but I think 4-HO-MiPT stands out at likely the best of all of them for a beginner for two reasons: unlike most other 4-substituted tryptamines which are powerful in a mushroom-like way already from low dosages for me, 4-HO-MiPT as more of a plateau division-like thing going on where higher dosages of it are similarly powerful to the others if not even more so but lower dosages are some of the most easygoing but simultaneously also euphoric and meaningful trips found within this entire class, and they have just enough of a psychedelic headspace to give you more of a real taste of what things like LSD can do than something like 2C-C would for me, but still not enough to be particularly confusing or disorienting particularly, just a fun trippiness. So yes.... 4-HO-MiPT, that would probably have to be my recommendation as well.
Now, I just want to address a couple things....
Not that it really matters much, but first of all, I want to point out that the commonly passed around wisdom that ondansetron directly blocks the nausea of psychedelics by interfering with their binding at serotonin receptors doesn't make any sense. As was already stated, ondansetron is a very selective 5-HT3 receptor antagonist, but to my knowledge literally no psychedelic except 5-HO-DMT, bufotenine, has been shown to bind to 5-HT3 receptors, whereas an extremely wide array of psychedelics including all the classics as well as decent representation from nearly every class of research chemical psychedelics have specifically been shown not to bind to them, and also not to 5-HT4 receptors, suggesting that it's highly unlikely that the psychedelics people are getting nausea from are causing that nausea via binding to 5-HT3 receptors. I'm not saying ondansetron doesn't work to help with that, but if it really does it almost certainly must be through an indirect mechanism, like perhaps the signaling pathways of gut 5-HT3 receptors also overlap with other 5-HT receptor sinaling pathways and thus blocking either one could raise the nausea threshold, but that's just one idea that comes to mind. This has been on my mind every time I've ever seen or heard someone metion using ondansetron this way and just thought I'd actually bring it up this time.
And second, I'd like to make the argument that the odds of MDMA being a genuine anticholinergic deliriant are, in fact, actually quite high. First and foremost, the PDSP did a specific certified wide-array binding study of MDMA, the results of which can be found in their Ki database, which only measured hits that were active below 10,000 nM. At this concentration, MDMA failed to bind to any of the monoamine transporters, including for serotonin, dopamine, and norepinephrine, but did bind to 5-HT2B at 700 nM, α2A-adrenergic at 2,532 nM, α2B-adrenergic at 1,785 nM, α2C-adrenergic at 1,123 nM, and H1 at 2,138 nM, as well as calcium channels at 1,198 nM, and on top of these, it also bound to M3, M4, and M5 muscarinic acetylcholine receptors at 1,185, 8,245, and 6,339 nM, respectively. In other words,
MDMA's most potent receptor binding affinities are for 5-HT2B, α2-adrenergic, H1, and M3 muscarinic receptors, and both M4 and M5 it at least has higher potency for than monoamine transporters. I have actually been looking into this myself recently because I have been doing more research on myristicin, the active molecule in nutmeg, which is a very close structural analogue of both MDMA and mescaline, and would actually be MMDA, the MDMA analogue known for producing particularly powerful "mind movie" experiences, if you replaced its allylbenzene tail with an amphetamine tail. Street wisdom often claims that nutmeg has both psychedelic and deliriant effects, but despite this I could find no scientific data whatsoever verifying that myristicin has anticholinergic effects and only indirect evidence that it can lead to the activation of serotonin receptors, so I decided to take matters into my own hands and run the molecule and some receptors through a receptor docking prediction simulator. As I personally expected, myristicin was predicted to have ligand efficiency at the 5-HT2A receptor similar to serotonin itself, and furthermore, it was shown to have nearly identical efficiency as a ligand at the M3 muscarinic receptor, but significantly less so at the M2 receptor, perhaps paralleling the lack of M2 or M1 receptor binding affinity measured for MDMA. Out of curiosity, I then ran DMT, psilocin, LSD, and mescaline through the same simulations and it was predicted that all of them were high efficiency ligands for 5-HT2A receptors, but DMT, psilocin, and LSD had low relative efficiency at the M3 muscarinic receptors, while mescaline in fact was also a high efficiency ligand there similar to at 5-HT2A receptors, but slightly less so at M3 relative to 5-HT2A compared to myristicin.
In other words, it would appear to me that the rough molecular structure estimated not only by MDMA but also by other similar molecules such as myristicin and mescaline may in fact be likely to allow for affinity to at least the M3 muscarinic receptor if not others as well in addition to their affinity at serotonergic sites such a the 5-HT2A receptor. I think it is worth considering that people have a tendency to look for certain familiar subjective signs in identifying deliriants and one of those is the horrendous body load, but from what I've always gathered from years of research I was under the impression that the worst physical problems they cause are related to blockade of M1 and M2 receptors, so I looked into the M3 receptor and found that its primary physiological role seems to be in activation of the salivary glands; from what I can tell, the main side effects you might be able to expect from blocking M3 receptors are things like dry mouth and eyes and retention of water from reduced sweating, the former two of which are widely reported and known to occur on both traditional deliriants and nutmeg and the latter of which is known to contribute to possibly dangerous circumstances from reckless abandon on MDMA and other related empathogenic substances such as 6-APB and other benzofurans which share this rather unique form of molecular structure. Another thing that people look for to identify deliriats is a heavy disuption of cognitive function,
but a selective M3 receptor antagonist in humans in fact either had no effect or enhanced cognitive function in contrast to hyoscine hydrobromide. Despite this, M3 receptors do exist in the brain in a similar distribution to M1 receptors and in areas such as the frontal cortex and hippocampus, so they are indeed positioned in a such a way that it's believable that significantly altering their activity could lead to mind-altering and/or hallucinogenic effects, and these would still be "deliriant hallucinations" despite the differences from deliriants like tropanes and diphenhydramine if they're all still caused by blocking the M3 receptor regardless, and even if there are other effects like those of psychedelics and empathogens running behind them.
This is all scientific and some of it quite theoretical, but I do have anecdotal evidence as well that helps corroborate it in my mind.... While I have not used mescaline, a full dose of nutmeg, MMDA, or any research chemical empathogen analgoues like 6-APB, there was a period in my life where MDMA and diphenhydramine were my two most commonly used drugs after cannabis, and I loved both them. At this time I was seeking hallucinogenic experiences on everything imaginable, including not only the known atypical hallucinogens like salvinorin A and muscimol but even things like purposing taking large doses of amphetamine and methylphenidate to experience the hallucinogenic effects specific to monoamine releasing agents and reuptake inhibitors. Furthermore, in all my time taking drugs period, I have used a total of fifty-two different specifically psychedelic substance and several other non-psychedelic hallucinogens. Yes, any hallucinogen pushed far enough will produce fully dream-like hallucinogenic experiences, but no, they do not suddenly become specifically deliriant-like in that way. I spent years chasing deliriant hallucinations because they were my favorite style of all, and I felt from the very start that MDMA produced deliriant-like hallucinations that were comparable to diphenhydramine despite obviously also being filtered through some other very different effects, and nothing else from the amphetamine-based hallucinations to any dosage of any other psychedelic I've taken so far has produced the same kind of experience for me and satisfied my desire to have the kinds of hallucinations that those two drugs have given me, and that's exactly why I was looking into nutmeg and myristicin in the first place....
Do I know that MDMA is a deliriant? I do not. But I do think it is? Abso-fucking-lutely. I think it is highly likely and supported both anecdotally and scientifically that MDMA has genuine deliriant effects through muscarinic acetylcholine receptor antagonism, even if its selectivity for that and non-selectivity compared to other neurotransitter systems makes them come through a very different overall subjective and physiological experience, and I think it's also quite possible if not probable that this deliriant-psychedelic/empathogen hybrid style is also shared by MMDA, 6-APB, and other structurally similar empathogens as well as myristicin, mescaline, and perhaps some of their close analogues. However, unlike some I might expect, that doesn't turn me off to them at all.... I specifically don't like diphenhydramine anymore due to its toxicity concerns and awful body load, but if I can get a genuine fun and safe deliriant with actual deliriant hallucinations, the more the merrier, if you ask me.
Anyway, that's just my view which I felt was worth sharing on all that.