1) Software development
2) I've done pretty much everything. DOCs are ketamine and amphetamines. Psychedelics were regular at one point but now a once a year thing.
3) Only one drug test in my career as a new hire for one company, and was using meth at the time. Stopped using 4 days before test. Except for weed which I dont use and benzos which I have a valium script that covers, almost everything is out of your system in 72 hours (assuming UA, follicle you're screwed). And ketamine/psychedelics aren't typically tested for.
As far as not making people suspicious, mainly comes down to knowing your body and limits; and adjusting your behavior to compensate for the effects. I would consciously speak and move slower, and question if anything I wrote was absolutley necessary and delete it if it wasn't before sending emails/IMs. You get unnecessarily wordy on stims.
Also would act tired occasionally even if I wasn't (more so around family, not at work). Made it easier to avoid conversations. However that trick might not work depending on history and DOC. My friend who used heroin in the past couldn't do that or his family would think he was on it.
A good portion of my career I could work from home, or worked for small businesses where the owner had their own recreational usage and didn't care. One job I became close with my manager and let him know (not advised), and though it didn't affect my job it did affect our friendship. Others, compensation methods I assume worked since I was never questioned.
4) Started experimentation ~7 years ago, was in my mid 20s and already had my career so I started with the mindset, but job and responsibilities always come first. If your performance starts to suffer, or if you're using an addictive substance and you start to go through a difficult time in your life (breakup, abusive relationship, death of really close friend/family) stop immediately. I ended up using as an escape at that point and things spiralled.
#4 and that part in bold specifically, if you take away anything from this, take that. It's what separates functional users from addicts with a problem.