check with flir for heat leaks & sources. Install vent fan.

Also spend the $$ to install a mini-split. Heat in winter, cool in summer, ability to dehumidify. Based on size of garage, i'd go with 2 smaller units, this way if one craps out, you're still in business.

All my out buildings (casita, reloading building, shop) have them. I have a few thousand into 4 units. But the peace of mind knowing you can work year round is worth it.
Where we're at is 4800' elevation, all 4 seasons, with over 100 on the rare summer day, down to single digits in winter.