I have two monitors and SDDM is prioritising my secondary screen: the cursor spawns on my secondary monitor by default, and my login splash screen only shows on my secondary monitor. I tried editing /usr/share/sddm/scripts/Xsetup as follows:
xrandr --output DP-1 --primary --auto
xrandr --output HDMI-A-1 --off
But it doesn't do anything even after rebooting.