I am rebuilding my garage/shed that appears to be ~50 years old.  It is a simple detached structure on an old slab.  I plan to replace with a very similar size/shape and reuse the foundation which is in decent shape.  However, I would like to make this a bit of an experiment to see how easy it is for me to insulate and airseal well (all my projects in the last 20 years have been renovations and I’ve never had an opportunity to really evaluate how good my technique is). 

The old slab has cracks in it but is fundamentally structurally sound.   It is completely uninsulated, however.  In my nerdish quest for good efficiency, I would like to insulate above the slab and tie that in with the wall insulation and air sealing layer.  Do you see any issue with my using the technique described in “https://www.greenbuildingadvisor.com/article/the-concrete-free-floating-plywood-slab-assembly” but instead of putting the foam over gravel, putting it directly on the slab?  The loads on the floor are modest and are commensurate with the approach in the link so I’m not worried about that.