And you're running X11, not Wayland, right? Just to be sure, can you check whether you can start the fallback (icewm) session (selected at login on display manager) with the same monitor configuration you've seen the problems on the installer? If this works on the installed system while not on the installer, something must be missing in the installer. Judging from hwinfo, nouveau is still used. So that should be same as on installer, at least...