The info below is from one of my replies to another user who had problems with SDDM scaling.
It is the “Wayland EnableHiDPI=true” that you might need. I forgot what I learned when I researched this a year ago. I do believe it carried over into the desktop, but I just can’t remember. You might check and see if HiDPI is enabled anywhere on your system.
Also, this post explains how another TW user fixed his monitor issues.
None of this may be related to your issue but you do have an odd monitor resolution and these might help lead you to the solution.
Make a file called hidpi.conf
Put this in the file.
[General]
GreeterEnvironment=QT_SCREEN_SCALE_FACTORS=2,QT_FONT_DPI=192
[Wayland]
EnableHiDPI=true
[X11]
EnableHiDPI=true
ServerArguments=-nolisten tcp -dpi 192
Put the file in /etc/sddm.conf.d/
Also, this post explains how to add video modes and refresh rates etc. My machine keeps waking up with the wrong resolution and while it will allow me to change the resolution back to 3860 x 2140, it will not allow be to change the refresh rate from 30 Hz. It doesn’t do it all of the time and may be kernel or driver related because the fix didn’t seem to work for me.
Still, it’s good info if you need to add a compatible video mode to your system.