Very high humidity in all rooms

, the OP needs answers about his options for action and not know-it-all questions. Interior insulation would be another alternative, 2-5cm of Styrofoam already make a big difference.

And now let's get started: Do you think airing out after showering is enough? Do you think airing out is effective when the outside temperature is, say, 5-10°C and 90% relative humidity?

Hi, after showering I always had about 90%. After airing out about 60%. But I think I need a proper measuring device that is reliable.

Once again the question to the OP (everyone else please don’t prompt answers :)) Do you think airing out once after showering and then leaving the house... is enough? Do you think hanging wet towels outside after showering and quickly running a squeegee over the shower/wet surfaces makes a difference? Would you be willing to install a simple humidity-controlled electric bathroom fan? Are you (you all?) able to air out more often and for longer?

I also wondered why the previous owners probably installed a simple (thin) interior insulation. My crystal ball says there were reasons for that...

Weren't there? You mean previously there was (even if rudimentary) interior insulation that you have now removed?

I don't understand what these leading questions are supposed to achieve. It has already been said that manual ventilation with new windows is not sufficient. Unless you consider ventilation a hobby. And if you start ventilating, then not only in the bathroom but also in the living areas. Ventilation alone - whether manual or automatic - will not solve the problem. Old building walls simply insulate poorly and the dew point can only be avoided with very dry air. In the long term, therefore, external insulation must be added.

Pedantic or not. Understanding the relationship between temperature, relative and absolute humidity has never seriously harmed anyone.

But the situation is basically clear here, right?
20°C room temperature is simply too low, heat it up to 22-23°C. That of course means 15-20% higher heating costs, I get that too.

We had exactly the same problem in our rented apartment in the winter of 2022/23 when we were all supposed to save gas. Previously everywhere was 23°, then I lowered the temperature in the study to 20°C and after 3 weeks mold appeared on the north wall. The 3 winters before with 22-23°C there was none.