Since you cannot move your setup to a short wall, here are two more things to consider:
1. Maybe you would have better luck with diffusion behind the listening position, rather than absorption, as indicated in your overhead plan. I have a friend who listens close to the rear wall, and the addition of diffusion behind his listening position improved his soundstage significantly. Since you already have diffusors in the room, you can easily experiment with them on the rear wall.
2. I just noticed that you have a vaulted ceiling. I once had a listening room with a vaulted ceiling, and it resulted in lots of undesirable effects, some of them unpredictable. I was able to minimize these effects by hanging absorptive baffles like these. Whether it would improve your soundstage, I cannot say, but it should improve things generally.
1. Maybe you would have better luck with diffusion behind the listening position, rather than absorption, as indicated in your overhead plan. I have a friend who listens close to the rear wall, and the addition of diffusion behind his listening position improved his soundstage significantly. Since you already have diffusors in the room, you can easily experiment with them on the rear wall.
2. I just noticed that you have a vaulted ceiling. I once had a listening room with a vaulted ceiling, and it resulted in lots of undesirable effects, some of them unpredictable. I was able to minimize these effects by hanging absorptive baffles like these. Whether it would improve your soundstage, I cannot say, but it should improve things generally.