This thread reminded me of my own garage conversion project last year. I struggled with that same hollow echo until I tried the acoustic slat wood wall panels from Decor & Decor. I started with just a few samples in walnut on the long wall and was amazed at how much it cut down on the noise and even made the room feel a bit cozier.
Once I saw the difference, I added matching panels on the opposite side and it really transformed the space into a liveable studio, and it helped keep the room warmer, too. If you’re after a straightforward solution that looks great and works well, those panels might be worth a try.