My garage is not heated and is great for cooling my beer if the kitchen refrigerator is full. I've tried everything to heat the garage easily, quickly, and to summer temperatures for painting and general comfort (while laying on my back on the concrete).
A compact, 4800w, 240v, "garage" heater does the trick. It was $100 at Home Depot. One of the best investments I've made. It also has a thermostat so it is not running all the time, otherwise my power bill would be crazy.
https://www.homedepot.com/p/Dyna-Glo-Pro-4-800-Watt-240-Volt-Electric-Garage-Heater-EG4800DGP/304663393
I can heat the garage to 70°F in 20 minutes. It's a beast!
I parked the heater in front of my car while I used rust inhibitor and oil-based paints to refinish the bottom of the car. I opened the garage door 18" to allow the forced air (and fumes) to escape the garage. In the following picture it was -4°C (24°F) outside. 22°C (70°F) in the garage. I love my heater!