First of all you can check distrobox.it which can basically run Neon inside your distribution however you better set a different virtual home for neon in that case.

I would first tar the .steam to be on the safe side but steam is different, it is some kind of Ubuntu stable itself residing in that directory. Not a big time gamer but people laughed at Ubuntu for shipping its snap because of it.

Long story short I don't think steam would have issues. I meant not to expect KDE guys to revert upgraded preferences back to KDE5 etc. You know they do such things and blame Linux/KDE etc.