Or perhaps the mushroom and fungi aren't the hazards at all. The nearly-sentient insect monsters that tend their fungal garden, however....
I was going to post along those lines, but got beaten to it.
There are several kinds of insects- mostly ants & termites as I recall- that cultivate/collect fungi to eat directly or to convert inedible things into food.
You could expand the list of sentients with a a culture of symbiotically sentient lichens. In one homebrew, I had elves who had used magic to make themselves more like plants, so they were green, photosynthesized, etc. You could do likewise with elves who have fused themselves with lichens.
Slime molds can become immense as they spread subterraneanly. So a sentient one might seem to have a godlike omnipresence in the environment you describe, especially if it can communicate (imperfectly) with other species. “Whispers in the dark”, visual hallucinations, etc. as its mind reaches out to talk to the beings moving nearby.
Perhaps a fungus’ spores can implant themselves in the flesh of the living, growing parasitically and eventually creating little ambulatory, violent “homunculi” that resemble their hosts. One part
Evil Dead franchise, one part “Seeds of Doom”, one part
Invasion of the Body Snatchers.