Problems of humidity in old buildings

So? Those are normal values. You have exactly those right now with a decentralized ventilation system.

Normal?

We always have over 60%.
And we are currently trying to ventilate at least twice a day every day.

My idea would simply be to install a ventilation system everywhere.
I could also widen two holes to 160-180mm.

Normal? We always have over 60%

Yes. Me too. Right now it’s 62% inside - by the way the same value as outside. After all, the temperatures inside and outside are the same. It’s physics and cannot be changed.

And we are currently trying to ventilate at least twice a day.

I do that continuously with a ventilation system.

My idea would simply be to install a ventilation system everywhere. I could also widen two holes to 160-180mm.

You can do that. It’s a good idea. But with the current values it will absolutely not bring you anything and will not reduce the humidity in the apartment. That’s a misconception. It only helps really well if it’s cold outside - read up on relative and absolute humidity. There are also online calculators where you can check how humidity develops at different temperatures. Then you will see that right now you have no chance.