Thanks for the tips!
However:
enabling CSM in bios (but still boot using efi)
That would also mean to disable secure boot, in turn causing other issues (security, functionality loss in Windows). I can for sure try that for a test, but could you explain / link to some explanation on why you expect this to change behaviour?
setting nvidia-drm.modeset=1 kernel parameter
That’s already the case, as you can see from the report:
$ zgrep nvidia-drm nvidia-bug-report.log.gz | tail -n1
root=UUID=32278c21-b19c-47e0-8466-420bbb5a1642 ro rd.dm=0 nvidia-drm.modeset=1 net.ifnames=0 pcie_aspm=force initrd=boot\initramfs-5.9.11-gentoo.img
Any further hint appreciated. If it’s better to also send the report to the nvidia bug report mail, just let me know.