+1 @fuzztone I think this could be solved with a simple switch. If you’re neurotic about sharing grounds, dig inside and switch the grounds (most switches buss the grounding). Make certain everything is off when you flip the switch - although at line level to subs you may be OK doing it live.
I would avoid using an AVR for 2 channel listening unless it’s a really good AVR. You would be missing out if you get ride of that tube amp before experiencing it.
You have the space. As others have said, I would set everything up for good 2 channel listening, fill in with the other speakers where they fit and use room correction on the 7.2 system. Spend some time with what you got.
A lot of this depends on your focus and what you use most. I have 2 systems in a shared space. After spending a bunch of time and money routing Atmos speakers and the like, I found two big issues. For starters, most of the film I enjoy is not in Atmos. Most good films worth the effort of the room were maybe done in 2 channel. Second, after experiencing a good SET tube amp, everything else sounds fake or artificial. The bouncing of the sound is usually down so poorly it feels like a gimmic. There is much more to a ‘real’ sound other than the physical location of the sound.