So I tried a few things, on arch, with Xorg and Wayland everything plays great. The jelly files, the previously tested files, everything just works. With wayland I can obviously not change resolutions within kodi, this is a known thing, but other then that it's great. GBM 'works' but is super unstable/unreliable.
With libreelec nightly (from a a few nights ago) I can play the jelly files from a USB drive, but with NFS being broken (at least then) I couldn't access any of the other files, so don't know if the issue is actually fixed on the nightlies.
I'll try again in a few weeks with hopes of the nfs issue being resolved from a live USB stick. But I must say that so far my arch experience has been smoother and more stable, which is odd tbh. Much less stutters, much fewer random crashes. But I still would much prefer running libreelec, as setting up the machine cost hours, in part because the 'max' resolution reported by my TV and being interpreted by Xorg doesn't work, but this was never a problem for libreelec. I'm using my .kodi from my libreelec setup, so all my addons and settings are the same ... Anyways, will report back in a few weeks time.