What tripped me up was the VisibleOnScreenNotifier3D node doesn't work at all in compatibility and the camera system I threw together for the jam relies on it. The more complicated system I planned on originally probably would have been fine but I didn't have time to implement it. Thanks for the tip on the srgb_to_linear()! I'll definitely keep that in mind next jam :D
For now I think I'm probably going to avoid building stuff in 4 for game jams since webuilds are such a big part of that. Hopefully they find a work around in future releases though, 3D in 4 is so much better than 3!