..this suggestion is harder to apply because I should think a way to change the plan of the first floor without messing up completely its design

Quite, I see your point. My suggestion was primarily made from a location where -20°C is quite common in the winter. But you can also look at a smaller fireplace. We had a modern one installed a couple years ago, < 70 cm wide and builds only 40 cm from the wall (chimney) - and it still delivers 5kW - more than enough for your nice little house. ok