If you’re using Wayland, you can go to Settings -> Colors & Themes -> Login Screen (SDDM) and click “Apply Plasma Settings…”
If you’re using X11, it looks like you’ll have to resort to hacky scripts, unfortunately.
Source: https://discuss.kde.org/t/how-to-change-monitor-layout-and-orientation-in-sddm/3377
Honestly, it’s probably not worth the hassle to try to install system apps elsewhere. Steam and Lutris, on the other hand, can put games on any mounted drive. Install them as normal, and then in Steam, set up a steam library in
/Apps
. In Lutris, you should have the option to choose where each game is installed each time you install one.