Underfloor heating air-water heat pump. House too warm when the sun shines

The 21° in the bedroom and dressing room are unfortunately also a compromise, the large bathroom is directly adjacent to the dressing room, separated by a sliding door. I have found in multiple attempts that when I turn down the bedroom and dressing room enough to make it really cooler, the dressing room then draws energy from the bathroom floor. So if I want the bathroom to be on average between 21.5° and (sometimes) 23°, I must not turn down the adjacent rooms too much.

At some point, however, I also reached the point where I no longer care about every single kWh without torturing myself and the family. Thanks to photovoltaics and storage, the past heating season was practically free, we have an all-in electricity purchase of 5000 kWh for 2019 and fed-in 6300 kWh. And that with 5 people in 2 complete households.
Of course, it would be nice to need even less... but maybe I should keep things in perspective.

Is there actually a guideline from when a room might be too cool and thus more susceptible to mold? I also like it cool when sleeping (<=18°). But in the new house, I don't want to risk neglecting the drying of the walls (Ytong) anywhere.

Photovoltaics or at least solar are something we also do, really great with your result

Please no solar thermal energy. The investment will not pay off in 20 years. Moreover, solar thermal energy complicates the efficient operation of the heat pump. You then need elaborate hydraulic modifications (which cost additional money) with corresponding control systems. Photovoltaics fit much better with the heat pump.

I did it like this myself, since the bathroom was/is the "problem area" for me as well. Larger heating surface means lower supply temperature required. I also did it in the guest bathroom; you can then really well dry the wet children's clothes on it.