Do we need the extra heat?
As this house needs more heat than the present Suryas can provide, I propose that it is worth expanding the solar capture and seeing how the tubes would perform. Let's imagine that the house needs 12,500 kWh annually for heating and DHW, and the heat pump is burning 3,300 kWh of power to do it, and the Suryas are putting down 3,000 kWh of solar thermal energy. That simple arithmetic suggests that 6,200 kWh has to come from the natural surrounding - that tells me that there is additional scope for putting down more directly injected solar heat. I also believe that there are system losses, for example the upper 3m of underground loop are not insulated and probably just lose heat to the cold upper crust. Some additional solar gain will compensate for those losses.