You need to set a budget because there are a ton of options, but you can spend 100k+ making a room if you want. Obviously the whole room inside a room is out, but ball park a budget so we know what kind of stuff to offer.
I used to jam in a concrete room the size of a bedroom. It was brutal and my band was loud. The walls and floors were covered in 30 year old carpets that I think someone got from the ends of rolls at a carpet store. They were super nasty so we decided after jamming for a year to remove them. This is when I realized they really alter the sound and kill some of the echo.
That echo is the sound bouncing around so you are trying to stop that with absorption. Building your own panels is cheap. Roxul Safe and Sound is the stuff you need.
Google sound absorption panels and there are 100's of videos on it
I put about 6 on my ceiling. The reason being I record myself and my overheads sound 100x better now. You have high ceilings so it might actualy not be as bad as you think in there.
Start with as much carpet, and hanging whatever you can on the walls. You want the sound to not bounce around evenly. Playing in a sphere would be a nightmare, followed by a cube, if that makes sense. As you add couches, shelves, things that are not 4 walls facing each other the sound will break up as it bounces around. There is alot of information online about how sound travels.
Build baffles and hang them above your kit, carpet, and hang stuff on the walls and assess.
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Those are also helpful, Like I said, I don't know your budget.
Find an office going out of a business and ask for that acoustically treated cubical stuff they have. Someone asked me once why that material costs so much, It's because it would be annoying as hell to hear an entire office on the phone at once. So that stuff kills sound very well.