[lsllc]

Given that both the desuperheater and the geothermal both need backup heat, the best approach is to install a small high efficiency gas boiler with 2 zones, one as an indirect heater for the desuperheater (so it'll ensure that the hot water stays hot, but won't run if not needed) and the second zone as a hydronic loop in the air handler to provide backup for those cold nights in the winter that are below the design spec.

Of course, the heat input to your house is a function of heat loss, so if the house is very well insulated and sealed against air changes, regardless of how you heat you'll save energy.

TBH, since geothermal is reversible, in that it does both heating and cooling this makes it even more attractive and efficient especially in the summertime. It should be required in new construction where possible and indeed heavily subsidized to encourage it's adoption.