(Edited for clarity) This was interesting. It gave me arandr to generate a script which is great for lazy me. That script "works" in that it doesn't give any errors when I test it, but it actually doesn't have any effect on the login screen. In fact with more digging i discovered that xrandr just doesn't work at all. I tried setting the display to a lower resolution (default is 3440x1440 so I used 1920x1080) in the control panel to test the xrandr command but xrandr tells me the mode (3440x1440) is not found. I looked again in xrandr and saw that any resolution higher 1920x1080 is not listed any more. I reset the resolution back to 3440x1440 in the control panel then looked in xrandr again and all the expected resolutions are listed again.

xrandr errors when I try to set my display to anything other than the setting it is currently using. Either I'm don't something stupid with the syntax (99.999% confident I'm doing it right), or xrandr is broken with my setup. Maybe kde plasma 6 and wayland is giving me grief here? My PC has an AMD 7900XT GPU, so maybe it just doesn't like my GPU for some reason.

Here is the output from xrandr for my current settings:

DP-1 connected primary 3440x1440+0+1080 (normal left inverted right x axis y axis) 800mm x 330mm
HDMI-A-1 connected 1920x1080+758+0 (normal left inverted right x axis y axis) 520mm x 290mm

Here are the commands I'm using in the Xsetup script.

xrandr --output HDMI-A-1 --mode 1920x1080 --pos 758x0 --rotate normal
xrandr --output DP-1 --primary --mode 3440x1440 --pos 0x1080 --rotate normal