The diagram helps for sure. You definitely have room for 2 rear speakers.
You could experiment with 2 on-wall M2s and put them as the rear 2 speakers, then swap the QS8s to the rear and put the M2s on the side walls to see if you like one configuration over the other. You will still want to bump the side surrounds (QS8s or M2s) just in front of your "head" as it rests during normal viewing.
Then grab 4 speakers for your ceiling and never look back. You will have a great space for Atmos.
I am a HUGE fan of room treatments. If you don't have them already, you will really love your room afterwards.
Another option for basstraps is to use 4" OC703 material mounted in a frame like if it was going to go on a wall, but straddle the corner. You will have 4" of great material, plus a nice air gap back to the corner which works almost as well as if you stuffed the corner completely full of insulation, but at a fraction of the cost. Of course you will need more than 1 piece of OC703 (theoretically two pieces per corner to get your 8 foot height to your ceiling).
Not sure if it is cheaper/easier than other options you might be looking at, but it does perform well... very well.
If you decide that you want to do something artistic with your wall panels, let me know. I am sort of the "go to" guy over at AVS for custom printed fabric DIY panels.
