I applaud your plan so far; for both summer and winter the high R glazing is the simplest solution (compared to higher insulating panels that must be moved and managed).
I have experience designing and building a home using sub-slab rock and earth as heat storage, and subsequent building experience with a home with insulated slab.
In my opinion the earth is overrated as a storage medium. The earth is likely to not reach seasonal equilibrium until a dept of 12 feet or more in your area. I had sensors buried, and even when the outdoor temps were above any upwards loss, I could watch heat being sucked out of my storage area below the rocks even when there was no loss above. I concluded that the earth is an endless heatsink except on much larger scale than I could build. In your area, I'd guess 55F down there at maybe 14 or 15 feet. You need to insulated both bottom and sides for it to work as a storage mass.
Earth is lossy and of piss poor thermal value compared to water. I now heat my home with just 800 gallons of insulated, solar heated water with a backup propane heater for the rare extended dark days.
If you plan to supplement heating, by thermal water storage or other means, I recommend you insulate the floor well, especially at the edges. Huge losses there, and the relatively poor performance of 30 inch wide soap bubble insulated high tunnels proves it, since they have outstanding glazing insulation value at night. The earth outside has a broad thermally conducive path to the inside, especially when the earth is damp or wet (and frozen). We live above ground so forget what is below, and heat does not.
If you set a hot frying pan on your hand, you will see that heat conduction downward or sideways works just fine, heat does not only rise by convection.
If you want to directly plant in the soil, the at the very least, get a trencher and bury 48 inch wide extruded polystyrene panels around the perimeter. That's still going to be a big loss downwards and outwards but more so in late winter as opposed to early.
Best Wishes
Bruce
