I tried a couple of different AMD cards, and my machine crashes on resume if I try to use either of them (but the Intel iGPU works fine).

Searching for amdgpu bug reports leads to:

https://amdgpu-install.readthedocs.io/en/latest/install-bugr...

which links to a page saying "Bugzilla is no longer in use" :-(

This is under Qubes/Xen, though, so maybe that causes extra problems. If any devs are reading, I did report it here in the end:

https://github.com/QubesOS/qubes-issues/issues/5459