Hi @cup-of-joe & co.,
I have been able to resolve my issue by logging into KDE/wayland, setting the screens the way I want to have them at boot (by going into Plasma Settings → Display Configuration), then going into the plasma settings → login screen (sddm), then there is a button top right which says “apply plasma settings”. Once I ran through that, the configuration is correctly used on every boot just the way I want, login only appearing on the 2nd screen.
Let me know it that helps in your case 
Kind regards!