I don't know if you have thought about this or not, but, how about just taking out the garage door and making it into a regular sewing building (aka sewing shed). You would have to rebuild the wall, but, might be easier to insulate and sheet rock, than trying to figure out what to do with the door. That or just insulate and sheet rock over the garage door, in case you or someone else would ever want to make it back into a garage again.
Whatever you decide to do, Good luck and let us know how it all turns out.