You can add some foams on the garage door, which will enhance the insulation. The overall temperature in garage would increase 6~8 degrees. But the CONs are, your cars, tool boxes and shelves are likely getting rusted much quicker.
It is normal the room on the garage is cold.
Especially, if the room faces north.
You should avoid to select model like this.
But I saw one builder has all models have main bedroom on garage.
Some builder allow to upgrade with thicker insulation layer on garage ceiling.
Now is difficult. Very likely you would need remove drywall to add more.
Try to add some insulation on garage door or replace it with double insulated garage door.