MicroAir does make thermostats for RVs with zone systems. I didn’t look at them or their instructions enough to know if you could adapt one of them to do something custom, like having a separate temp sensor in the basement and use the thermostat to respond for it as well as the living area.
My plan at the moment is to run the main furnace when the temps go down to around freezing. That’s what I always did with the 1685 and never had issues (but then I’m rarely somewhere that it gets below 30°F). I’m treating the electric heat the same way that I did when I used the space heater, which is essentially what it is. It’s just bigger and uses more electricity.