This house came with a 400# solid wood, double door. When I finally could afford it I replaced it with a fiberglass one that was under 150#. I don't want to go back to a heavier door.
Insulated doors work both ways. When you park the car and close up the garage then all that heat is trapped inside. Before you suggest, for security we don't leave the garage door open unless we going/coming or working in the garage.
I never leave mine cracked open either. A lot of people around here do. Especially in the hot weather. Some, like my neighbor across the street, even open them in the morning, and leave them wide open all day long until they go to bed at night. There is no way I would do that. It's just asking for theft. The overall property crime rate around here is pretty low. But with the schools all closed until God knows when, there are a lot of kids running around, bored to death. Many are just looking for mischief to get into. Out of sight = out of mind, as they say.
Before I had these new doors installed, I was considering those insulation kits they sell at Home Depot and Lowe's. They basically are a bunch of Styrofoam panels you cut and fit into your existing door panels. I believe once you get them in you apply some type of adhesive to keep them from moving around, and / or falling out. They sell for around $70.00 and up depending on size, amount of panels, etc. The better one's have the insulating foam backed with foil.
I'm glad I went with what I did. The inside panels look clean, with no gaps. If you decide to go with insulated doors, (or any new door for that matter),
shop around. Prices on these thing fluctuate wildly. On 2 doors, (double & single), I found as much as almost $1K difference. I ended up going with a local guy, who was recommended to me by the people who painted my house. He was an excellent guy to deal with. He came right over after I called, measured everything and gave me a quote. And he got the doors in 4 days earlier than he said. And he installed them the very next day. And he never asked for a dime until the job was finished. Everyone else wanted deposits. Some wanted 50% or more.
Stay away from these national chains like, "A-1 Garage Doors". They charge as much or more for a non insulated door, than I paid for insulated one's. And both came with all new tracks, hardware, rollers, and springs. Some don't. Just new springs. It's like buying anything else these days. If you're not careful, you'll get clipped.