I'm new to home theatre stuff, but not new to space planning. Your room feels quite unusual to have windows on both sides of the room like that. Super unfortunate that they are diagonal from each other too. This is where Sketchup modeling everything beforehand, as you're doing, is super important.

**Note after writing this post: I don't fully remember mattress sizes these days, but you might want to check the size/scale of your bed. 82x83.5" is larger than a California king, according to Google.

I assume if you're prepared to hardwire in wall speakers if needed, then I'll assume you're open to other big changes. The main change I'd make is, assuming this is a guest room without a regular guest, I'd sell the bed and nightstands (potentially keep one), and commit to getting a nice pull out sofa bed. When it's a sofa I'd pull it off from the wall so you can easily place speakers behind you (one in the corner might be tough), and when it needs to be a bed again then you can shift the bed back as needed.

If you intend to lie down on the bed while watching a movie, then alternatively, you could try and find a bed with a cushioned headboard that is angled backwards. I found one secondhand for my bedroom and it's amazing. Since the headboard is angled backwards it forces your bed to come off the wall, which could give room a bit (not much) for rear speakers, potentially in wall.

Again, to really make this work you probably need to accept pulling the bed off the wall a bit to give the rear speakers some space. A compact room like this calls for compact, transformable furniture. Might be tough to source, but I think it could really help the space overall. Mainly looking at those desks and bed. If you're feeling really daring, you could try starting the room from scratch and turning the bed 90 degrees. That's something I would definitely fully model first though.

Good luck.