This is bad information. The precursors being legal doesn’t mean anything about the legality of producing scheduled drugs from them. The precursors for home distilling are also legal.

Possessing schedule I controlled substances is illegal. If you grow the substance, you also possess it. Therefore it’s not legal.

I've bought grain spawn cubensis bags at head shops before. Super easy to grow.

Do be careful. Depending on the state mushrooms become illegal at different stages of production.