IMO, you are on the right track. Reflective film, like the old movie clich'e cop sunglasses, will reflect an amazing amount of heat away. Good, long lasting film will go on the inside, and though it would take a razor blade and a very long time to remove, it can be removed and replaced if damaged or you no longer prefer it. But during the winter, it also reflects the sun's warmth away. But that is certainly a trade I would make without issue, as I prefer to have the cooler summer.

 I have very limited experience with insulation, so I best not comment much on it. However the reflective film, I do have some experience and it has been all good experience. I've been around houses that have hung roll up bamboo shades in front of their windows for the summer. I wonder if you can do that in front of your garage door? Any time you can keep the sun from directly heating up a surface, it makes a lot of difference.