Alright, good news. After 12h and renting a mac with the new ARM architecture (silicon), I was finally able to figure out the problem.
The new unity version added building for that architecture. The default is building the game for both intel and silicon. While this is generally a good idea, the problem here was that the webp libraries were not compiled for silicon, which caused them to not be loaded with a generic error that could have meant anything.
Now, the new macOS has a thing called rosetta, which enables apps to run that were compiled for intel. Without that they would have lost 99% of their current apps.
Long story short, the trick was to switch the build to be intel only, since that was what the old unity version did by default, so there isn't anything lost here.
Eventually, the webp wrapper will officially release silicon libraries and then I can switch over.
I will also be fixing the slow quest menu. I have more optimizations already done, but I want to break things again, so I'll do more testing on that.