Sorry I didn’t pay enough attention to your picture now I see the speakers drawn in. I think you are a great candidate for at least 7.1 with that room configuration. With your seatin 12 feet back I think wide speakers would help but not as much as rear speakers would.
My room is 13x21 so close to yours. I sit 15 feet from the screen but only 12 feet from the speakers as their drivers are about 3 feet out from the front wall. I’ve tried only 5.1 in this room and found that moving to 7.1 really helped fill out the 360 degree sound field. Since it looks like your surrounds are fixed at about 12 feet due to the right one I really think that you would find it worthwhile to use rear speakers.
Personally I prefer using QS8s for both surround and rear speakers but if your closest seat to the rear speakers is 10 feet I think you could use M2 or another bookshelf speaker as a rear and save some money.
Because of the distance to the mains I find that wide speakers also help make the sound field more enveloping but it’s not as noticeable to me in a long room as adding rear speakers is.
