LP gas heating water for a baseboard hydronic system in the house. Similar in the shop, except that the hot water flows to some radiator units and fans blow the heat into the rooms.
I'd like to look at solar hot water for the shop heating duties but it's getting tough to find contractors that will work with those systems since PV is becoming cheaper and cheaper.
That sounds practical (although looking at the picture you posted I was instantly thinking wood stove, haha; just a romantic association I guess). I believe solar water heating has been going out of style for a longer while now, but I can see that making use of your existing radiator system would be something to consider in your case.
As it happens, it WAS wood heat before we got here. : ) Really big (750 gallon / ~2,800 litre) outdoor boiler that heated both the house and shop. But he was filling the thing up top to bottom three times daily in the coldest weather. That wasn't going to work with my traveling job, leaving the wife at home to do that. We yanked it in the process of replacing a failing retaining wall, and added the solar PV nearby the following year.
Our challenge is in siting the collectors, given the terrain and trees around the place. Given that, I think our next step is to go nuts on weatherization of both buildings. Reduce the demand, first.
For the happy stuff, my back problem seems to be muscle spasms, and it also seems to be getting a bit better.