It’s hard to say what is right or wrong without knowing how you will use each room. 20 is a lot to start out, most will buy 1 or 2 rooms and add on from there.  There is nothing wrong with your setup on the surface though. You can move things around to see what works best for you, and return what you don’t need.

I might do a pair of Fives in the master bedroom rather than dining room, and maybe have more used rooms with a pair of Ones instead of singles...but again,  it depends on how you use the space.

Looks good to me. I would consider changing the speaker in the master bedroom on the second floor from a One to a Move. This will give you a little bit more volume for the larger room and allow you to take the speaker outside.

Also the two Ones being used as surround speakers for the Arc in the living room can be One SLs instead. When the Ones are used as surround speakers, it disables the microphone. So save some money and get the One SLs for the Arc.

In my opinion SUB is not in an optimal location. Subwoofers should be near the plain of the front speakers (ARC). An obvious position would be near either door on the TV wall. Another possibility is under a piece of furniture near the TV. Finally  if you can, I suggest that you mount the surround speakers on the wall. If you have the surround speakers sitting on tables, the person seated in the center of the couch will have proper surround. A person seated at either end of the couch will have one or the other surround speaker dominate. Wall mounting is simply an excuse to keep the surround speakers a little more distant -- minimizing dominance. If you tend to listen solo with your head at one end or the other of that couch, you could use Trueplay to optimize for that location.

Keep in mind that SUB needs power.

Dear Danny, GuitarSuperstar and Buzz,

thank you so much for your replies! (I don’t know how to respond to you individually).  I really appreciate it very much that you took your time to help me. I didn’t even expect such thorough advice. These are really good recommendations.

Thank you so much!

Sending greetings from musical Warsaw - Poland ;-)

Basia