How many sq feet of diffusion do I need?

It's 9 x 10 ft. and not meters, right?
First question is, how does the room sound to you now...is it a "live room," meaning if you clap your hands, is the sound reflected like an echo? I can see that your walls are probably reflecting sound.
Secondly, what are you hearing or not hearing from your rig; ie; is bass too boomy or not tight? Are highs harsh or do they sound natural? You can first play with speaker position and toe-in.

In summary, how does your system sound to you right now?

I think you mean they found some Nodes. I would do the same as you, do it in incremental steps. First of all, you need absorption on the side walls (the point of first reflection where sound from spkrs bounces off).
On the right side, definitely on the door and maybe a strip on the wall next to door. The left wall maybe 4x4' or 2x4 vertical. BTW, you actually need some reflection in all rooms.

Also, something on the REAR wall behind listening position will absorb standing waves. Room treatments such as these will make such an improvement; after that I would work on whether or not you need bass traps. AND I think you should shop around for the right kind of panel; foam would be much cheaper than the one you posted.

Gotta tell ya, you have a very nice system.

Right on, Krell_man. I'm using the same setup as you in my small room, except side walls are vertical.
And your point is correct, he needs absorption, not diffusion. Panels can be flat foam, wedged, but not 3D. I'm using 2" offset wedged.

examples

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.