I am fortunate enough to have an outstanding audio space. It was acquired quite by accident. The best my dealer/friend has heard in thirty years of installing high end systems, many are $200K+ systems built for the purpose. Unfortunately, it is the exact opposite of what you are planning.
First mine is underground. The quiet is palpable down here. Your plan is to subject the entire structure to vibrations and external noise by putting it above the garage is just a receptor to sound and vibration from the outside world.. My space is asymmetrical in every respect. Basically it is on the top half of a very large Z room. See my UserID to see my main system. The three basic things that makes my space great is: underground, highly asymmetric, and large. Another great thing is that the main bedroom (my partner sleeps during the day) is upstairs and far away from my listening space. She is a light sleeper and cannot hear my system no matter how loud I turn it up.
I am sorry to say, but you are completely on the wrong path to having a great listening space. The stud spacing is never going to make up for location, symmetry and size. I would reconsider everything.
I am really sorry to be so blunt. Your forthought is fantastic. I wish I had such great forthought… I did not… I was lucky.
Humbly, George