Well first important thing is getting the desk in a better place. You want the desk to be centered on one of the walls. Next thing is to get your monitors pointed at you and not lined up straight with the desk. Your ears should create a perfect equilateral triangle with the speakers. This alone will give you a more accurate sound.
What I added in is an ideal way to start acoustic treatment in a room, but there is some stuff i couldnt add because its 2d. You also want to cover first reflections on the ceiling. Also, since the back wall is brick, you may want to consider adding a few extra back absorbers so as to kill that wall just a bit more. Although killing the sound in the room isnt the goal, but brick is particularly reflective.
There's also the chance that the door and window might be right where your first side reflections are, so you may have to be sneaky about that one. For windows....well personally I'd cover the window just like its a wall, but for more aesthetically pleasing of a look, you could build something that could hang there over the window whenever you want it to. The door....yes that needs foam on it if its in the way....but again it may be a bit strange looking.
The bass traps are misleading to many. The mistake I see a lot of people make is putting the bass traps wayyyy up in the top corner, but you actually benefit more from it being treated more like a first corner reflection. This means that it will be at speaker height in the corner. You also may need more 4 bass traps. In that case, I suggest putting them somewhere else and not just stacking them in the corner. Just always make sure its a reflection point. Meaning that if you put a mirror there and sat in your chair like you were gonna do a mixdown, you could see the speaker in the mirror (for corners you need two mirrors).
Buying foam is pretty expensive, so don't get too excited unless you have about 500 bucks to drop on a roominators kit or something. Building is not a bad idea, but you have to do some research as to what material is ideal, and you have to know a thing or two about carpentry.
Oh and I didnt even get into soundproofing or diffusion. Those are usually more time consuming or money consuming additional benefits. Same with power conditioners which are like surge protectors that are designed for sound people. Believe it or not, but a regular surge protector creates a good amount of noise.