Your issue seems to be that Xorg still try to use your nvidia card while having no suitable driver for it. Check dmesg in dom0, you will probably see drivers errors saying that 4090 is not supported; it can be useful to debug. You probably need to configure satan xorg.conf to force it to not try to use nvidia card. Try that GPU Passthrough -> lightdm.service won't start - #20 by light it could help. Create a Gaming HVM .
I had the same issue that I solved on similar hardware a year ago, but I don’t have access to it currently and don’t remember what I exactly did