I’ve got heat pumps and they work fine as a primary. They keep it at 70 for us year round with minimal fuss. I didn’t pay an extra huge premium for any fancy model either as far as I know. Newer ones work well into single digits very efficiently and still work into lower temps than that, but not as efficiently or quickly. Having a backup source for the coldest nights is a good idea but not a necessity. Heat pumps and air sealing/insulation work would be my solution for the OP.