My dream for this would be lots and lots of rockwool filled panels on the walls, at least 4" thick, with a sound transparent false wall in front - could be as simple as some wood framing with fabric stretched over it. And as others have said, drywall off the garage door area to make a rectangular space on both sides, one for the room, the other for storage.

And some suspended absorbers from the ceiling (since you said it’s already finished) to tame reflections. Again some simple wooden frames filled with rockwool would work and be cheap.

Possibly some bass traps in the corners but they don’t do much below 100Hz unless they are either enormous or active (eg expensive). If I could afford it for subs, I’d have an active echo cancellation setup, eg a load of subs at the front, and a load at the back, DSPd so the rear ones completely absorb the incoming sound and eliminate standing waves. It’s been done in the pro audio space, there’s a video on Youtube somewhere - but they did use about 24 subs in total!