Hi there, I was building a similar spreadsheet to yours when I stumbled upon this "heat pump balance point" tool that Efficiency Vermont put together. It's much better than anything I could have come up with. Check it out.

"This Heat Pump Balance Point Tool was developed to help contractors find the proper balance point based on system capacity and economics. The balance point is the outdoor temperature at which the heat pump no longer has the capacity, or is no longer the most cost effective option to heat a building.
By entering data into the input cells on each tab of this simple spreadsheet users can find the outdoor temperature at which a dual fuel thermostat will change heating systems."

https://www.efficiencyvermont.com/trade-partners/heat-pump-balance-point-tool