If you're going to keep the heat on in the garage, I don't think you can overdo it. There are doors that are rated at over R-16 insulation value. They also tend to have better seals at the joints and edges.

Thanks for your input.

I’ve not known of radiant floor heat in a garage. For sure insulate below the slab prior to pouring. We reside in N. IL, our overhead door is R18 rated with insulated glass. Similar with the Service Door. Walls would be R13.

How warm do you plan to maintain the space? We keep ours at 45 degrees unless working in the garage. The 35,000 BTU heater warms it quickly.

Definitely pay attention to the seal, a small air leak can undo all the insulation.

In-floor radiant with a well insulated garage is common in MN. You need to pay attention to both the R value and U factor. A high R value with also a high U factor (you want a low U factor) indicates a lot of thermal bridging - the cold/heat is basically going around the insulation. IIRC our doors are R-20 / U-0.18

We have R-40 ceiling, R-27 walls and R-35 under the slab. Insulating the interior side of footings will help eliminate condensation on the floor (this s/b done all all basement footings as well). See photo near bottom: https://bamasotan.us/2018/05/stripping-waterproofing-i/