Crystal meth was discovered in 01893; it has been legally available by prescription in the US since at least the 01950s. I don't think it's ever been entirely illegal like marijuana. Nowadays its main legal use is for obesity treatment under the name Desoxyn.

What drug would be next to target on the list of "good candidates for decriminalization" after marijuana?

In order of ascending net harm to individual and society: Mushrooms, buprenorphine, LSD, Ecstasy, Steroids, Khat

https://www.aei.org/carpe-diem/what-does-science-tell-us-abo...

Personally, I think a great amount of harm reduction could be achieved by legalizing at least one recreational drug in every class, determined by which has least relative harm to the user. If there was a clean, legal and affordable option, I believe demand for the more-harmful drugs in the class would be significantly reduced. i.e. "stimulant" demand is somewhat elastic, and by making the least harmful stimulant (not sure if that is Khat or not) easily available, demand for Meth would be significantly reduced and market effects would make its availability much less widespread

I don't know enough to judge and I would hope that we'd look to the science, medicine, and what other countries have done with success to make that determination.

Psilocybin is the first possible candidate that comes to my mind, but I'd rather have us use a sound framework of harm-minimization than to favor/disfavor any particular drug.

Psychedelics are probably next. Already there is some movement, Oakland and Denver have decriminalized magic mushrooms.

Also the entire state of Oregon has both decriminalized, and has approved and is setting a legal medical infra just like with marijuana.