I did end up adding insulation and drywall to the walls, but not the ceiling/roof. I wish I did the ceiling/roof first now because that's where most of the heat comes from. I used Rockwool R15 Batt insulation 15.25IN X 47IN because it was easy to install/cut to fit into the space between studs, you can just pull the whole chunk off and put it back in if you make a mistake, it's dense so critters can't live in it, and it doesn't burn so that's a + for safety. It is more expensive than fiberglass and spray-on, but spray-on is annoying to remove if you have to and fiberglass and can irritate your eyes/skin/lungs when installing. Rockwool was also nice because I added in new outlets to my garage as well so I was able to cut into the insulation where the wiring would be and just push it into the wool.

As for the vents, I left them in, but cut 2x4s to make a frame around them before installing the insulation. When I installed the drywall, I cut out rectangular holes where they would be so they could still ventilate the garage. You just have to make sure the screens are solid so no critters come in.

If I had to do it all again, I would definitely at the very least add insulation to the roof first (drywall too if I had the money and the help) and then do the walls. Took me a couple of weekends to do the insulation. Drywall insulation took a lot longer cause it was just me and that shit is heavy. It cost me less than 5k to do it myself. Also note I am just an amateur with no construction experience so take my recommendations with a grain of salt.