For sure run the wires both for 6.1 and 7.1 since the cost will be minimal compared to everything you’re doing.
I’m with Adrian that rear speakers just may not be worth the cost/effort. Have you run a 5.1 system with the QS8s with the setup you have now? If not give it a chance before committing to 6/7.1. In my 21 x 13.5 x 8-12vaulted room I’ve switched back to 5.1 for now. I have no dedicated 7.1 sources so everything is DPLIIx and find no improvement using 7.1 over 5.1 for movies or music, and 7.1 actually makes most directional cues in games sound much worse.
Now I do have about 7 feet behind my seats and place my surrounds further back than most which probably allows for a better rear soundstage to develop from the surrounds than 4 feet would.
Problem you might encounter with 6.1 that close is that “rear” sounds may seem to those sitting on the left/right of center to be coming from the side and not the rear. So 7.1 may actually work better since it will spread the rear sound source out across the back. Maybe someone with a similar setup will chime in with real experience. Even then the only way to know if it will work for you is to give it a try. You can always order a couple rears try them out and ship them back if you decide it’s not worth it.
Two QS8s will have no problem filling the space behind you with more sound than you will want. IMO your best bet is to start out with 5.1 and if possible experiment with the surround’s positions. You may find you really don’t need 7.1 right now and later when more discrete 7.1 material becomes available if you’ve pre-wired you can easily upgrade.
Your Denon should have no problem delivering 6.1 from 7.1, 5.1, 3.0...
