My brother-in-law also has only 2.10m on the ground floor in his renovated cabin. After a week’s vacation with him, I don’t feel like I’m missing anything (regarding the ceiling height).
My great-uncle also lived during his lifetime in a small house with about 2m ceiling height, which today would no longer be considered living space (maybe not really even at that time, but he didn’t care). But the man wasn’t 1.50m tall either. That still leaves more than 50cm of free space above his head. For me personally (~1.72m) visiting there was no problem, but for my husband who is another 20cm taller, that would already be quite uncomfortable.
So, if we were to look at something like the property here, the ceiling height increase upstairs would be 100 percent a given and must be taken into account; otherwise, you’d have 1.92m body height and 1.96m ceiling height in that area of “Honey, don’t forget to take off your slippers before you go upstairs” and putting on a T-shirt is only possible sitting down or limbo dancing. With 2.10m to 2.15m ceiling height, I still have in the back of my mind that I have two cousins around 2m tall (one just over, one just under, but I’m not sure if the 18-year-old is fully grown) and who knows how tall one’s own children will be and if that might already be uncomfortable...
(Yup, I have a huge range of heights in the family... one side is tiny, the other populated by giants ;) interestingly, my parents are practically the same height, but in one family branch that is unusually small and in the other by far the tallest)
edit: I can’t contribute much to the heating technology, but I find the topic interesting. My knowledge is basically that radiators can work with heat pumps, but probably need to be replaced by larger, more extensive ones.