I have a similarly sized " music room" but it houses a half bass at the moment not a piano. I would put the piano under the small windows where the chairs are and a small sofa on the long wall and small chairs on window wall. This was my inspiration room

this is my sofa, bass and chair setup there is a small scale wing chair behind each sliding door angled towards the sofa and my bass and table take up around 5 ft of the 8 ft wall.

I would consider moving the piano to the other wall and perhaps going lighter on the walls rather than darker. Perhaps a wallpaper on the other wall as well or a stencil. Then perhaps a loveseat and chairs on the other wall and in front of the windows. A black and white pattern might be something to consider.

I would use a warm white paint to make it cozier, my fav is Marscapone. If you like gray better try BM Smokey taupe. It's grayish tan.

To counter all the cool tones and to make the piano look less heavy, I would suggest an Oriental rug that is mainly red. You need curtains panels on the windows, and I would suggest a cream color in a heavy, possibly self-patterned fabric. Move the piano to where the chairs are and make a conversation area with a couch in front of the windows (great light for reading), a small coffee table (glass to show off rug), an end table with a nice lamp and a small easy chair or two across from the sofa. Definitely beautiful art on the walls