I know this thread is a bit old but have you considered framing and insulating the basement?We‘re on a slight grade, so about half is below ground. One wall is against the garage and another is open to the backyard. Luckily the open side faces south, so that helps in the winter.
You are right, though, the temp is a lot more stable than upstairs, especially in the summer. It usually doesn’t get much above 70F in the summer, is around 50F now in this relatively mild weather, and into the upper 30s when it’s in the teens or lower outside. If it gets close to zero or below a heater MUST be run, sometimes in more than one section of the basement.