adding the kernel parameter:
amdgpu.dcdebugmask=0x10
Seems to do the trick. I’ll have to check whether software runs fine for me, but there was no glitching in greeter/login screen, or when changing X11 refresh rates or Wayland resolution
edit. I’ll write a “solution” post after I test this
edit. I haven’t tested everything yet but this is good enough, no glitching on greeter/login, both X11 and Wayland display DE fine without need to refresh cycling or lowering resolution. SuperTuxKart and Witcher 3 are both playable. Double monitor setup works without problems.
Thanks to @Dinomonster for sharing the solution