I usually prefer having any side machines running something more stable than the main one, as I’m always bound to use and mantain them less often.
Good luck finding something more stable than Debian tho. Maybe something like LMDE, that just got a new version out and is looking great, or trying out an immutable distro.
I’ve never had this as an issue with KDE. Do you have the command for prime render offloading on the Steam launch options? I usually launch my games through Lutris and it handles that pretty well.