I added insulated garage door that I think was r17 for an attached single car garage thats insulated. Since the door is metal, went with more insulation since even aluminum will lower temperatures. Really only need a small space heater to comfortably work in there during the winter, but fuck its hot in there during the summer. Having a smaller heating/cooling is needed if your going to be in there consistently if I thought we were staying long term. Even without a dedicated temp system, it's night and day from the years without an insulated door.
If it was a detached two car garage, I would go with the best insulated door I could find thats reasonably priced. Buy once and don't have to pay double down the road to replace it.
For the trim, I had my wife hold a side of the trim with the lights off in garage while I'm inside for the 1st piece. If I can see light, then move it closer. I can also tell from that what i should shoot for distance wise. Figured the more light, the more cold air that gets in. Take it with a grain of salt