Lots of good advice so far. Definitely would do 2 subs in that space. They will see all of the available airspace so 2 should be good for around 6db extra and help smooth it out as mentioned. Also agree with keeping the surrounds more to the sides than rear will be better, and 7.2.4 might still be possible.
When we built our house, I needed a place for my right surround. What we came up with was a pillar about 14x14 to mount on, and also hold the light switch for various areas of the space. My wife hates it but I was not going to build a house and not have a spot for one stupid speaker lol. It’s not THE most elegant solution but it works.
Anyways, I just wanted to throw it out there that maybe a small(???) architectural detail might solve one of your issues. Maybe not, just putting stuff out there. Here is a couple shots for reference. The old room had a 35lb jbl speaker mounted on the pillar. We redid the room and I used in walls for side surrounds(part of my Atmos compromise lol). So now the room has 7.3.4 installed. Still waiting on avr...