Very high humidity in all rooms

Goods? You mean before there was an (albeit rudimentary) internal insulation that you have now torn out?

When was the first occupancy? Is this a detached single-family house or what is it? Note: regarding the humidity values. Relative humidity always depends on room temperature and outdoor weather conditions (temperature/humidity/season/...)! This means that in summer, with usually very often opened windows, the values are only a reflection of the prevailing outdoor conditions = normal = from 30% to over 90% depending on the weather, all unproblematic = normal. In winter, heating and ventilation behavior/user behavior is very important. With underfloor heating, surprisingly often there is incorrect user behavior... especially if there is no experience with it. Regarding ventilation behavior, the question arises: can/will I ventilate sufficiently often? Or is no one home during the day and the wet towels... the wet shower... slowly release moisture throughout the day into the room air/walls/furniture... after the morning (only apparently sufficient!) ventilation.

We moved in in 2019. It is a detached single-family house. Actually, here in our area this year there is generally very high humidity. Currently between 70 and 90%. We try to regularly ventilate by opening the windows wide. After showering, we open the windows all the way until the humidity goes down to 60%. We now have a measuring device in all rooms.

Do you think that airing out helps when it's, say, 5-10°C outside with 90% relative humidity?

What’s the point of the question? 100% humidity at 10 degrees results in about 55% at 20 degrees. So it helps a lot after showering. And otherwise, you can’t really do anything wrong. Of course, you can’t expect miracles in autumn weather