The forum helpfully suggested a few related posts below and particularly this one was helpful: it suggested plasma on X11 can still be installed as a fallback on Fedora. I have done so and indeed my compositing troubles seem to be solved for now. I can even use hardware acceleration again in firefox.
So the pressure for me to get this fixed has been removed. But I would very much like to be able to use Plasma on Wayland eventually so if anyone has tips to debug or fix this, I’m all ears still!