Pure DXM powder is still best to eat. Simply too much would be needed to snort, it'd be massively unpleasant to snort 100s of milligrams of DXM. Sublingual is out because DXM tastes bad. I've heard reports of people plugging DXM and they experienced extreme and painful diarrhea, plus apparently it burns quite a bit, which would also contribute to the unpleasantness of snorting it. I've never heard of anyone attempting to IV or IM it, but I'm not even finding much about it online at all. I feel like there is likely a reason for that.
DXM is a unique drug, while it's very much a dissociative it is a unique one and the only drug that I've ever taken that was close was MXP, but while DXM is stimulating, MXP was sedating. I wouldn't say that low doses of ketamine or PCP (though i've only tried analogs) are like DXM. DXM can be just as powerful if not even more than ketamine or PCP, it just depends on how much you take. I was never any good at describing the more abstract effects of hallucinogenic drugs, so maybe someone else can compare them in more detail but DXM vs ketamine, some of these are going to be subjective and vary person to person as well. Ketamine is more recreational. It's more fun to dance on. DXM is more unpredictable. The existence of plateaus is unique to DXM. Music sounds better on DXM, imo. Actually I'd describe DXM as the best drug for music appreciation. DXM tends to have a dirtier feel to it, though that is absent when dealing with pure powder. Actually a lot of the side effects like nausea and vomiting have more to do with the method of ingestion than the drug itself, though DXM does come with some level of nausea. DXM lasts much longer. DXM can provide a much more profound trip. DXM feels more like coming home.
I'm not going to do PCP because I haven't done it and could only really compare it to the analogs. While I hear 3-MeO-PCP and PCP are very similar and the biggest difference is just that PCP is less controlled, more chaotic feeling, I don't want to speculate.