Pros and Cons of Ventilation and Exhaust Systems

...we can only manage that in the bathroom if more than one person showers immediately one after the other!

And this moisture goes away again through consistent ventilation...

We live in an old converted calf barn, so the insulation conditions are rather not so good.

Stuffy air and mold are not directly related.
Damp/moldy walls develop as follows:

Indoors there is a high absolute humidity. Due to missing insulation or other thermal bridges, there are lower temperatures on the wall or window (below the dew point, can be calculated).
Here the water in the air condenses, the wall becomes damp and eventually mold appears.
This can also occur in the window reveal with permanent tilt.

That is why many window manufacturers, for example, also say that windows in old buildings should not have too good a U-value, because otherwise it will mold. But this is not due to the U-value, rather it is because the old window was practically ventilated by force.
And on the new, tight window the water then settles and not on the wall.

Mold therefore depends not only on ventilation but also on heating. Especially in this constellation it becomes critical in new buildings. A solid new building certainly takes a good two years until the introduced water is gone.
If this meets incorrect heating behavior, it quickly becomes critical.

And what would be the correct heating behavior.

I also know the window draft here in our rental apartment, built in 2000, 17.5 lime sand brick and about 18 cm insulation.

I just hope it will be better with only aerated concrete without insulation.

I just hope that it gets better with just aerated concrete without insulation.

The material doesn't matter at all. The house is just as airtight with aerated concrete and the walls are just as thermally insulating. By the way, with aerated concrete and generally monolithic construction you are more likely to have problems with thermal bridges, especially at window connections and ceilings.

Better wet windows than wet walls. But both are only caused by incorrect ventilation and ideas like "I don't heat my bedroom and at night the window must be open."

...Stress with moisture only occurs due to incorrect ventilation behavior, insufficient heating, and construction defects!

I have never experienced that so much water accumulates in a heated and well-insulated house that it needs to be removed.

The phenomenon occurs when, for cost reasons, the living space cools down during the day in the heating period and the heating is only briefly turned up in the evening.

The wall and inner pane are then too cold and the water condenses on them instead of being absorbed by the warm air and carried out by ventilation.

This can be revealed well with a laser thermometer...

It is important to ventilate the bathroom to the outside and not into the adjacent living space!

So here in the rental apartment, it is definitely not heated too little. And still everything is damp in the morning in winter.