Also, I'd add more bass trapping. The 2 in the front should be full sized, 2'x4' traps. If you can add more of them in wall/ceiling corners. Cover as many corners as possible. RealTraps - Acoustic Basics
I also agree with centering the mix. Just to add, I would make sure you are as close to the front wall as possible to keep you from sitting in the middle of the room (front to back). Most of the time one of the worst spots to sit. Also you really are going to need a lot more trapping in corners to tame things AND I would put THICK broad band bass traps (around 6" with a airgap) and or something tuned (around 70hz) on the back wall. Honestly though due to the size of your room you might just stick with broadband GIK or Realtraps?
+1 to everything already stated. More bass traps. Wall Ceiling Corners are just as good as vertical ones.
From the diagram it looks like you could place the desk in the centre of the opposite wall. This would avoid the door entirely.
Trunk no, stuffed couch probably useful.
The window could be helpful on the front wall also. No big.
If you need to keep the incoming light stay as you are.
There are higher performing bass traps designs than the simple fibre.
Some of these should do wonders. My Experiment with a Metal Panel Absorber