Well, your room is small but it can be treated. I wouldn't worry about the rear wall symmetry. But I would trap the heck out of it. You have a lot of space in the corners that you can use for trapping.
Note on sharp corners: Corners are excellent places for trapping! If you cut them off, like some crazy people would have you do, you lose the ability to trap there... Right there in the junction of two boundaries! Right where your traps will work TWICE as good.
That said; I recommend that you trap the rear wall straight so that the corners are deep and the middle area is about 8" thick. That should get you off to a very good start. You'll find that your phantom center image is more stable and focused.. and once you get the ceiling treated, the RT-60 will be so much better that you'll actually start hearing where the sweet spot is... and where it's not. 😉
Then... once you get the entire room treated, you'll be able to pace while you mix and things won't sound so different in other areas of the room. 🙂
No need to worry about diffusors in this small room. I think you're too close the back wall for them. But you probably don't need them.
The door on the left will probably need at least a 10 cm deep panel on it to prevent early reflections from there. In fact, I would treat the side walls and front with at least 10 cm broadband trapping. You can put slats on the front wall.
With every bit of treatment you apply, you will experience an improvement in your listening enjoyment. It's not going to make a shitty mix sound good, but at least you'll be able to hear that it's shitty! LOL!
All the best!