You can toss a mini-split in there even with no insulation, you're not looking to cool the place 24/7 or keep it running, you just want to be able to turn it on if you know you're going to be working on the cars. Back when I went to the track often, I had garage work down to a science - before you go, bleed brakes, swap pads, change oil, filter, check diff and trans fluids make sure they aren't burnt, drain radiator and put distilled water in it. Go to track, swap tires on before first session, drive all day, swap tires back, come home, pack car away, next day I'd swap the pads back.
I would have *KILLED* for a mini-split in the garage at that time just to turn on while I was working, it'd be a lifesaver. And mini-splits are CHEAP. Look into it!