Which insulation technique should I use to insulate my subfloor?
On a similar project, I put down 2" (?) foam board insulating panels, then filled the rest of the cavity with Rockwool bats.  Inspector was happy.  Here's what it looked like from the outside, before the wall was finished (opening used to be a doorway)



Edit 1: Added picture of filled joist spaces

No, I filled the entire cavity

EDIT 2

Original plan called for (from the existing slab upwards) crushed stone, 6 mil vapor barrier, 4" concrete slab, up to near the top of the foundation wall.  Then the sill plate with the floor joists on top of that.  Final step was filling the rest of the void with closed cell foam insulation.  We deviated a bit because of the added cost of the stone &amp; concrete.