First place to start is the figure out the best dimensions and shape of the room. Everything else is secondary.

Typically a square room is terrible because any standing waves will be doubled. Perhaps add a secondary “room” parallel to the existing garage door that would serve as a storage area for lawn equipment, bikes, etc. and would also change the listening room dimensions to something conducive to good sound. You will also need to probably adjust the ceiling height or modify it to have a slope or varying heights.

By the way, I am totally jealous.