I like your efforts so far and expect the results to be an order of magnitude beyond my garage / workshop. It's the never-ending project I work on between other priorities in my life.
Like you, I did the pex in the slab and extruded poly insulation under and around the perimeter of the slab, just not as heartily as you. I did 3/4" under the slab and 2" around the slab inside the perimeter of the block foundation walls. I did not go to the effort of filling the blocks with the beads like you did. What I am getting around to, slowly, is painting the blocks outside {100% completed} and inside {15-20% completed} and plan to glue 2" of the extruded foamboard on the inside face of the blocks and go over the exposed foamboard with some sort of waterproof finish. I haven't made up my mind on the material to use, yet.
One last thing, I sited the building as close to true south as I could get away with and added plenty of windows on the most southerly wall and a carefully planned overhang to block out the heat in the summer. I'm hoping the passive solar does the vast majority of the work heating the garage.
Wish you luck and I'll be lurking.
Mark