It seems like GDM is still loading too fast, even with the early KMS. This kind of reminds me of an issue I had with Wayland on Gnome last May, except in my case GDM wouldn't start at all.
You don't have to read that whole lengthy thread; the "fix" was to make a drop-in file for the GDM service with sleep 3
added so it would just wait a second before launching. Give it a shot, it may resolve the issue for you too.
Set up a drop-in file for the GDM service:
sudo systemctl edit gdm
Add these lines:
[Service]
ExecStartPre=/usr/bin/sleep 3
Save, exit, reboot.