Boy, that sounds like the hard way to make a thermal mass, especially in light of the already present concrete. I’m also concerned with how you are not insulating the bottom of the “dirt thermal mass”.
Here’s a quick science question. You want the largest thermal mass possible in a given space. Your choices are a) depleted uranium (freakishly dense) or b) plain old tap water. Which would perform better? Answer is b) tap water. Thermal “mass” has nothing to do with actual mass.
Dirt is a weak thermal mass, and concrete isn’t much better - but more importantly, both are rather poor thermal conductors. Water, however, is an excellent thermal condutor and the best thermal mass known.
You’ll want these plants set up on shelves anyway for ease of gardening, so here’s what I propose.
Lay thick layers of insulation on top of the existing concrete; this will be the subfloor of your greenhouse. Come up the sides with also very good insulation. Everywhere except the aisles, put tanks of water. Say, 55-gallon drums stood vertical and packed dense. Set the planters on top of those.
Packed all in heavy insulation, that water will hold heat like crazy. In fact you’re betting on that because you don’t want the water to freeze (though you can certainly use non-toxic propylene glycol antifreeze if needed).
All this doesn’t require digging, hard construction problems, or modification of the concrete slab.