The garage floor won't be as critical for comfortable temps as a floor in the house. In the house a floor temp above 80-85* give the people a 'hot foot' treatment.. it's too hot. In my ceramic floor'd bathroom, the parts of the floor that are about 75-78* are the most comfortable, only once in a while does it get above 80* and I do notice it. Especially in bare feet.
In my shop, I keep the floor at 55* or so, this makes for a comfortable 45-50* air temp, just fine for most work. Since the slab takes many hours to change temp, I don't try to raise the floor temp for a half-day of work, I just add supplemental heat with a propane radiant heater aimed at my backside at the work bench.
Makes a nice hand warmer too.
My shop has 16' ceilings so getting the air a lot warmer is expensive. but with my feet warm, I'm warm. The little added radiant does the job. All the walls are insulated to R13, and the ceiling is R16, it is 2" foam with aluminum sheet facing down into the room, it reflects the radiant from the floor back down... it really works, you can feel it.
If you want warmer temps, then raise the floor temp the hottest I've ever had mine was 65*, a 55-60* air temp in the shop and toasty to work in.
You can buy tempering valves or mixing valves or you can make a manual one with several ball valves and a diagram to control the mix of hot water with recirculated floor water. I use a plate heat exchanger, 'cause I have a glycol mix in the floor, 'just in case'. I'm away for up to a week several times a month.. Can't loose sleep over a freeze up.
Hope this answers your questions... feel free to ask more.