Hi ethicist. I would recommend insulating the walls with at least R19- if possible-and using that insulated garage door.
And here's a tip-
In the garage ceiling , -get pricing on having a cell foam insulation sprayed in there-. I've built many, many living spaces above garage projects here in New England, and the same condition always occurs-even with the R38 fiberglass batt installed correctly and tight to the floor underside, it eventually allows cold air to collect between the batt and your heated space subfloor. The cellular foam sticks directly to the subfloor, helps to keep them feeling warm underfoot. I know it will seem a little pricey initially, but trust me, you'll know it was a good call the first frigid week with that lake effect wind blowing. Good luck!