Let me first say that I am not an audio purist, like many on this forum and ML owners. Just like balanced clean sound and not at all interested in chasing after the ultimate sound quality.
For Atmos, I use Motion 4i speakers mounted on the wall at the ceiling height (9.5' ceiling). Installing ceiling speakers, albeit ideal, was more than I wanted to take on at the time and this was a good compromise, The ribbon tweeter on the 4i are the same size and unit as the rest of Motion series (unlike the 2i) and does pretty good job matching the sounds of EM ESL (mains). Since you have 20s and 30s, it should match exactly, if want to consider the option. The surrounds are Motion FX and SVS-2000 Pro sub. The sub is ported (yeah I know) and provides great LFE for movies. I can plug them for music but I have found that as is works for kinds of music I listen to (mostly jazz).
How do you just plan to switch btwn 7.1 and 5.1.2? I realize you can simply move the connections on the receiver (especially if you have banana plugs easily), but how about the speaker positioning between 5 and 7 surround locations?
Since vast majority of steaming contents are 5.1 or Atmos, the only real practical 7.1 source is via UHD BR disks. That said, most of recent UHD BR are encoded with Atmos or DTS-x so while 7.1 may sound better than 5.1, my sense is that 5.1.2 would add more 3D sounds than 7.1, with little to no compromise for sources without Atmos/DTS-x.
I realize there is a room configuration variable that plays into setting up any sound/speaker setup so my experience is strictly based on the room I have.
Hope this helps.