Adding poisons (fumigation) is definitely not a good idea. In mushroom plants the compost/humus used to grow mushrooms is often steam boiled to sterilize it, to keep the yields high and the production safe from any dangerous contamination. It is seeded with the spores of the desired species afterwards.

you could probably autoclave it with a standard dental/tattooing autoclave (~500 USD and requires a gas stove)

Pressure cooking in small batches is the diy standard, I've had good results with a standard insta-pot