My garage wants to be a studio!

[JWL]

Good plan for the most part. Definitely avoid the triple leaf thing, though if it were my room I'd consider taking down the plaster, turning the existing shell into a single-leaf, then build a separate room within the room.

Regarding HVAC, I'd also look into a Mini-split system. About $1kish, relatively easy to install, and will not compromise your isolation. Downside is that they don't provide fresh air so make sure you can regularly open windows/doors to get fresh air into the space (which will then need to be cooled down again). Not the most efficient system in the world, but very cost-effective.