Just in cases someone comes across this thread and still desires per screen scaling under X11:
I use multiple monitors scaled differently in X11 by taking scaling control away from KDE and setting the scaling manually with xrandr.
Under System Settings → Startup and Shutdown → Background Services I disable:
- KScreen 2 and
- GNOME/GTK Settings Synchronisation Service.
Then I use xrandr to set the scales I want:
xrandr --output DP-4 --auto --output DP-0 --auto --scale 1.75x1.75 --right-of DP-4 --mode 1920x1200
I set this at login by including the above in /etc/X11/xinit/xinitrc.d/50-xrandr.sh.
I use the above to run a 4K screen along with a FHD screen by scaling the FHD.
Of course this is not a officially supported KDE configuration, but it’s worked fine for several years. Should anyone want more info, the arch wiki has quite a good write-up about multihead: