What I'd do, and it's very easy for me to spend your money....

First, get that garage insulated. Do the maximum amount of insulation that can be done in the walls. Check with a couple different contractors, specifically insulation contractors. I assure you that whatever you do now, in 30 years that will probably be considered insufficient. I personally don't think you can over-insulate. That includes that garage door.  Buy a high-quality insulated model. It will be heavier, so you will probably need a new opener mechanism.

I'm not big on window film. Rather, I'd suggest as others mention here, heavy drapes instead. When closed, they keep out either sun or cold drafts, whichever you decide to control. Film can alter your view when light isn't perfect. Don't know if you put up Christmas lights, but film might affect what those outside can see of your lights.

Good luck.