Oh, you need a design wall! Just kidding, kinda.
the foam core insulation boards (they are sometimes light blue in color) are a good form of insulation. I know you have the room already painted, etc., but I would add this board (you can use screws/nails or even a dot of glue in several places around the outside of each board, then tape the joints with the plastic tape meant for that purpose. I'd at least do the wall that is part of the garage wall...the garage is probably not well insulated. I know the light from those windows is woderful, but a film over them to block the sun's rays will make a big difference, and you will still be able to see outside. Even a light color, uv blocking drape would help...but I like being able to view the outside...so, would go for the film.