Geothermal has advantages, but air source is getting so good that it's really becoming a niche.
Spending $5K on insulation or heat recovery ventilation will be more effective than spending it on a hole.
I saw an awesome home refurbishment in Montreal, they just went all-in on insulation. The heating was just done with a 500W resistive heating coil, just for the coldest days. They didn't even have a heat pump, except for the heat pump boiler. The heat recovery ventilation did the rest.