I think Jay J has you covered on the insulation. You already have plenty and more than most. Any heat in the garage will only help upstairs and not hurt but economics depends upon how rich or poor you are. Some poor souls work out in the garage in coveralls without any heat because it isn't economical for them.
I think you need a local hvac pro to look at your system. I have a concern that there is no return for return air. Short of leaving doors open, you need a return to get any air flow through the room. The person should also look at the long runs. Any type of duct fans or damper adjustments need to be carefully balanced.
You can test the "air flow" in your house with a tissue test. With the furnace running, hold a sheet of Kleenex up to each return vent. See if the tissue is sucked up to the vent and held or not and also watch the force of the suction power when the tissue sticks to each vent. Do this for each one around the house and that will tell you generally how balanced everything is or which rooms are getting more heat. Also crack open all doors (upstairs, downstairs, and basement) to see if the air flow closes or further opens any doors.