But since we’ve already got the fictional construct of subspace, the notion of a mycelial species that can extend through it seems…within the realm of truthiness, all things considered.

Especially since TNG already had creatures that lived within subspace, in Schisms. If humanoid-ish beings can live in subspace, fungal life doesn’t seem that much of a stretch.

The part I’ve never fully grasped is how one travels along the network, but then, I’ve never fully grasped how the warp coils are supposed to work, either.

IIRC, it’s like the Hitchiker’s Guide to the Galaxy Hyperdrives. Once you get into spore-space/hyperspace, you get an infinite amount of choices to navigate through, but if you can figure out how to figure out your path, you can exit where you want. Though unlike the Heart of Gold, which tests every single possibility and impossibility simultaneously, the later iterations of the spore drive take a bit more after Dune, where a navigator can commune with the mycelial network and divine the way the ship should go, rather than needing inordinate amounts of computing power to brute force the solution.

How the ship is moved along the network after the navigator figures out the route is left as an exercise for the reader.