Well you do have options that will fit in with your SO requirements.
Were it me I'd look into in wall speakers for the fronts, lowering them a bit. I'd wall mount the TV to move it up some for a better, imo, viewing angle and place the center on the credenza.
I'd to these things before spending money on an amp that imo won't help. Your speakers will be driven by the avr easily and will free up more money for better speakers that will improve the audio on their own.
Unfortunately, imo, this wall is not a well thought out plan. The fireplace is placed poorly and seems just jammed in to say there is a fireplace. But you do have options.
Take your time, check out options for speakers, etc.
If you do/are able to go with inwall speakers you can paint the grills to match the wall color further "hiding" them in the room without lowering audio quality. A win for everyone.
Good luck and keep us informed.