Welcome to the community @Downer_Dan 
Here are a couple of other topics you might look at for context and details:
Perhaps you might try switching off your second screen, just at SDDM login, seeing as only one will work at the moment when using the Breeze theme (established bug). Something like this:
/usr/share/sddm/scripts/XSetup
#!/bin/sh
# Xsetup - run as root before the login dialog appears
xrandr --output DP-2 --primary --auto
xrandr --output DP-1 --off