I'm a couple degrees south of you, and a lot of degrees east of you. I've never lived through a winter in your area, so that's understood. Earthships are designed for and work decent in the four corners area, if I was you, I'd be moderating that design for your climate, less mass, less glass, more insulation. Also, in floor heat, GRAVITY, wood fired I'm guessing. 16" concrete is mostly wasted for thermal storage and strength. Arch that back wall and you could get away with 4-6" reinforced concrete back there, that would be more than adequate for the solar gain you'll get and economically store. The soil behind the concrete can sub for the extra concrete, neither one will do squat day to day. For your climate, look up sand bed seasonal storage if you're still considering seasonal storage, that is, thicker than a couple inches from surface. you can hook the wood heat to that, unless you're way sunnier than I'm assuming, you'll be using the wood heat, and the storage will make it easier to use.
Or forget the concrete and build a block wall, much easier than forming, mixing. Or if you have lots of rocks, slip form the walls with stone layered with concrete/mortar.
repowerguy, what do you think of this one? decent price? probably make a $1,000 difference if it was closer, but it's the range of what I see advertised. Call your local redimix place is probably the best answer.
https://wausau.craigslist.org/hvo/d/minocqua-1985-advance-fdm-mixer-truck/7188365381.html