> the best way to keep the area warm in the winter.

What approach do you want? You can shore up the room to prevent heat loss $$$$$ or you can oversize a heating system to keep it warm $$$?

One involves a GC to help, the other a HVAC contractor.

I have the same issue in a very old house. The best long term is to get a GC to tighten your building envelope but the most costly upfront. The most economyupfront is to just oversize a unit but not economical long term.