How many sq feet of diffusion do I need?

Not to flog a dead horse, but couldn't you hang one vertical panel on each side wall as seen in Krell_man's System photos. It looks like you have room for a panel to the left of the door or hang it on the door.
IMO, that would absorb some standing waves; your exposed side walls are causing major reflections which lead to increased brightness.
That's it from me, I'm not trying to sell you anything. lol.

Tommy,

RealTraps and GIK Acoustics, and probably others as well, have stands for their absorption panels. Then you can just move them when you need to.

Chuck

I would try absorption over diffusion. I believe an acoustic panel on each side wall, even if not ideally placed as to 1st reflection, and base traps in corners behind speakers would help. In addition, you will have done something which seems to be a need for you. Check out ATS Acoustics, I have made my own panels and base traps from their supplies.

I am still having fun experimenting with my listening room.
Regards, Mike