The heat that goes into the ground is less than you would think. I live in the Northern part of Canada. Frost usually goes down 6-8 ft here. Slabs are always poured without insulation under it to keep the frost out. Insulation is vertical at the walls and then out horizontally.
Up here in the Northern part of Alberta, detached structures are almost all built with a Monolithic slab with no issues. And our frost level goes down 6-8 feet. You just need a good base to start with
I live way way up North of you guys. In the Northern part of Alberta(directly East of Juneau Alaska). Up here, 90% of garages are a monolithic slab with a thickened edge. They work very well. But that is probably to do with the fact that all the garages here are heated in winter time.
I picked up an old Lincoln "Tombstone" buzz box for a 100 bucks. It's put together many projects over the years. I always say its the best 100 bucks I ever spent in my garage