The shape of my room is similar to yours. I'm using four large subwoofers. I have limited areas to place my subwoofers. Here's what happened in my situation and I think it will help you. At first I had three subwoofers on the front wall. One subwoofer on the inside of my mains right next to my equipment rack. The third sub was in the corner where your sub is. The fourth sub was on the right wall just outside the corner firing into the room. This placement caused to phase issues with my mains and center. So I reset the subwoofer distance to get the phase time aligned better with my center channel and mains. I had a guess that the sub on the right wall was still out of phase and causing some cancellation issues. So I stacked the fourth subwoofer in the corner on top of the third subwoofer. This was the perfect placement. Once I did that the bass was even better. It was tighter even though both subwoofers are corner loaded.
I would suggest that you put the larger sub on the outside of the left main. Then stack the other two subs in the right corner. I have a feeling that those two subwoofers being that far down that right wall facing each other is causing some phase and cancellation problems.