Another suggestion. Do you have any insulation between the slab and your feet. You can add some insulation by building a sub-floor, but this will take away from the ceiling height. You can add electric heating under the floor. One advantage of this method is that when your feet are warm, the room temperature matters less.
I second this. If you don't mind redoing the floor, add an inch or two of XPS rigid foam on top of the slab (taped but not glued down), OSB on top of that, and then your choice of flooring. It will make a world of difference, especially if your house is air sealed.
If your house *isn't* airsealed, do that first. Otherwise, you will get what's known as a stack effect (
https://en.wikipedia.org/wiki/Stack_effect). Hot air will escape from the top of your house, creating negative pressure that sucks in cold outside air into the bottom of your house. Stop that air from escaping, and you'll keep the temperature a bit more stable.
And, of course, there's always a mini split heat pump. A single head unit on your bottom floor will keep it toasty, so long as the air isn't escaping outside. The operating cost will be much cheaper than a space heater, but the installation cost will be more.
Start with air sealing, then slab insulation, then supplemental heating.