I received a couple of used vinyl windows which I hope to install this summer. There is one small (14"x14") one installed in the back wall.
Haha, on the "guest house" idea.
I built a rocket stove heater (without the mass) and it has been a treat to have since I can heat the shed by 5° C every 30 minutes, even starting from -11°C (the coldest it ever got in the shed even with -20°C ambient).
There is R22 in the ceiling and the south (long wall) with R14 in the rest of the walls, all rockwool.
I realized that the ground was giving up heat into the shed, which got me to wondering about whether how to insulate the footing. I've been thinking about digging two feet out from the footing and installing 1 1/2" x2' pink rigid board horizontally below grade. But I realize that the footing needs to be insulated as well, and I just have to figure how I'm going to do that part.