The solution to close all heating circuits on the upper floor and only open the radiator in the bathroom is simply wrong. Strangely, nobody writes that; instead, they keep going on about the good KfW55 insulation values.
Open all heating circuits, adjust the heating curve and flow rates, ERR on, ÜSV on. But others can explain that to you better. I myself am still working on optimizing my heat pump, and have been doing so for 1.5 months now.