Was it Snow Crash or Diamond Age (or something earlier?) that had mushrooms as the basis for advanced technology? I'm curious if there was actual insight there or a happy coincidence.

It wasn't quite fungal, but Diamond Age had the Seed. The Seed grew any material/structure/device desired as opposed to the Matter Compilers and the Feed. We know that the Seed required tons of computing power to design and create, but the exact method of function of the Seed wasn't detailed. We do, however, know that it may have been nanotechnological as Nell had nanotechnology that her Mouse Army created to counter the nanites in the Drummers.

Not advanced technology, but I recall Piers Anthony wrote about intelligent aliens.. https://en.wikipedia.org/wiki/Of_Man_and_Manta (1968-1972)

The Stone Sky series by NK Jemisin referenced the use of genetically modified fungi as self-assembling construction material. Really cool to see how pervasive mushrooms are in sci-fi and how there's lots more potential to unlock.

I don't recall this in snow crash