Your biggest sonic issue with that room is the windows. Get heavy drapery and the ability to open / close them. For watching movies and listening to music, you want the windows covered by drapery. Multiple layers are OK as well there.
"Stuff" on the side walls are beneficial. Bookcases, pictures in frames, LP or disk storage open to the room (ie not drawers that hide the software),knick-nacks on shelving or some kind of storage bins / bookcases, sports or movie memorabilia, whatever odd-shaped and varying density items you can think of, place that between you and the system on the side walls.
I don't think ceiling treatment should be looked at until you have taken care of the reflections / absorption on the back walls and side walls.
You can buy room treatments, but generally they are expensive and not very effective.
You can do just as well by investing in the things anyone would want in a comfortable feeling room. Surround yourself in things that speak to you. Let the wife go nuts if you want; after explaining to her what is needed so she doesn't spend money on inappropriate decor.
You are doing well by having space behind the listening position, but some form of absorbion on the back wall might be helpful. Maybe hang a large-ish persian rug, build a diffuser panel, put an interesting object hanging a small distance from the back wall (in the past, people hung tapestries ... think about a modern take on that theme. Maybe a perforated leather cowhide, rawhide straps to a curtain rod, who knows. Use your imagination; your movie / music room doesn't have to look like a recording studio).
The best, reasonably cost effective ceiling treatment I know of, is to buy a rug, something simple with a natural backing like jute, I'd go to the rug store and get a remnant / end-of-roll quality example for low cost. You don't need much.
Then take it to an upholsterer, and tell them you want random length, full width throw rugs, say 6" to 12" in length, and up to your room width of 14', with serged or binded edges (binding is the least expensive).
If you had somewhere between 5 and 10 examples, you would be fine. Hang them from the ceiling, maybe two to a few feet apart, random value there also helpful, or a repeating distance, say 18" apart. Your method of hanging is up to you ... they can be right up to the ceiling or with a space of a few inches from the ceiling.via grommets or eyelets ... the upholstery shop will punch them for you as well.
Consider whom the tallest person likely to use the system would be ... with your 7.5' ceiling height, maybe you would want to limit the largest size to 10" if you intend to hang them 2" from the ceiling, for example, so that a 6'4" person would not hit their head (people's gait is higher than their height).
You want to focus on the first one third of the ceiling. Leave the rest as is.
Attached a general view of a music space, as seen from the top of the room, and the use of ceiling diffusers:
