Obligatory I have never insulated a garage door, but I do like insulating things.

Replacing with newer glass could work if the company that makes your door makes replacement windows and you can get ones with more panes or something? Otherwise might be kinda tricky, unless they’re a standard size.

Is the rest of the door insulated and are the edges properly weather sealed? Even though the windows probably let a lot of heat through per area and they’re harder to work with, the whole door is a lot more area and easier to insulate so could try starting there. And of course weather sealing will keep you from exchanging as much air.

If keeping the windows working as windows isn’t too important, you can probably cover them with the same stuff you would cover the rest of the door with, seems like people usually use foam and some kind of radiant barrier, often integrated in one product. If you do want the windows to work, then maybe the cling film type products that work for windows would work? I could also see bubble wrap or something working pretty well for a cheap improvised alternative, especially the kind with the big interconnected bubbles that’s no fun to pop.

Anyway, I hope one of those ideas is helpful and you can insulate your door!

Unheated well ventilated garages are better for your car. So I would insulate the wall between house and garage, and stop heating the garage.

Garages have motors with liquids that produce fumes (aka voc in the air). If you are insulating the garage door well, what are you doing to make the air remain breathable if a motor starts to leak? An explosion or poison gas in your house (if you have an attached garage) may be worth putting off air sealing.