To make it less small, consider knocking out a wall. The one between the drums and keyboard would really open things up. But for real, I’m not sure there’s a great way to do that.

You might want to look into building some sound absorbers to put on the walls. Don’t but the acoustic foam stuff, it doesn’t do much and is expensive. You can get semirigid fiberglass boards fairly cheaply and wrap them in fabric, then mount them to the walls. There are a bunch of tutorials on YouTube.