1. The main problem I imagine is not consent but ethics approval. You are probably gonna find some experienced psychonauts curious enough to want to try, but getting it approved by an ethics committee I imagine being tricky.

2. Multiple times per person over months risks attrition which gonna mess it up, and the statistical power increase you get could be offset by that (eg if a person gets it they may not be motivated to come again for the placebo). On the other hand ppl may be more motivated if they know they are gonna get it at least once so dunno.

3. Placebo with hallucinogens is always tricky. I would use psilocybin as (extra or only) control, esp if you test for little ppl. This way you can test if they report more tiny ppl in the new mushroom vs psilocybin.