Building land in the middle of nowhere with the house prices?!

Life in the village is different from in the city. You have to be aware of that. Does it suit you yes or no. Even if it does suit you, there is still the commuting. A lot of time is spent on a journey that is for nothing.

At the latest when the screed is laid, ventilation is necessary... 3-4 times a day. That is already a challenge when you live 3 km away as the crow flies...

This is one of the smaller problems compared to all possible issues. You can use fast-drying screed, which does not require ventilation. Or you can ask a neighbor who already lives there and only has to walk across the street. Driving to the construction site a hundred times just for ventilation is not necessarily required.
Appointments at the construction site are quite stressful alongside work, especially if you have to travel a long way.

I commuted by train for 2 years, 3 days a week, one hour each way before Covid sent me to full remote home office for the time being.

Yes, commuting by train is more relaxing than commuting by car. But don’t underestimate the stress factor anyway. Trains get delayed or canceled entirely and that can have various impacts on your work and personal life (especially if you only commute for "important" appointments and otherwise work from home). In the long run, that still takes a toll on your mood.

For example, I had a 10-minute walk, 40 minutes on the ICE, and 5-10 minutes by bike from the station to home. During the 40 minutes on the ICE, you could even work through emails, create presentations, etc., which was quite nice.

Still, I would never go back. Your children are a bit older than ours (0+3), but there is so much unexpected stuff that is simply hard to organize with that kind of commuting.

We also had the discussion of country vs city. We even had a plot of land in the countryside (only 5 km outside, but poorly connected) and still decided against it. We are simply city people. Sure, sometimes when I ride my bike through the villages, I find it really beautiful and quiet there, and it would be relatively affordable. But in everyday life, it’s rather the 5-minute bike ride to the secondary school (for us, getting up at 6 a.m. or so would be the worst punishment and the bus in the countryside was really impractical in the morning…), the 5 minutes on foot to the sports center, my wife’s short commute (less than 10 minutes by bike), and so on that decide.

(The Swiss have already convinced me not to do Saturday shopping anyway)

My husband was also a weekend shopper, but also because in AUS the stores are open on Saturday and Sunday. I quickly broke him of that habit here, or rather he broke it himself when he saw what it's like in the stores on Saturdays. I can hardly stand the crowds and prefer to go during the week. In our village, the Aldi - if you show up an hour before closing, it's wonderfully quiet there.

if you show up there an hour before closing time, it is wonderfully quiet.

That’s true, but the fresh produce section is then also cleared out. At least in Berlin, you hardly get anything then, or whatever is left has a reason...

Your tips may work for a limited period, but not for years.
Especially since I just thought of another point. What do you do with your children?
At the moment, daycare, okay. It runs from 7 a.m. to 5 p.m.

But elementary school starts at 7:45–8:00 a.m. and finishes at 12 noon. There might also be an after-school care, but it closes at 4 p.m. With an 8-hour workday plus lunch break and 3 hours of commuting, you are on the move from 7 a.m. to 6:30 p.m. Both of you?

And then shopping on the way home, making dinner, playing with the children... there’s no time left for anything else. After a year at the latest, you need the whole Sunday to recover, leaving at best Saturday for activities.

That is an insane workload. If you have to because it can’t be avoided… for example because you lose your job and only find a new one 100 km away, okay. But to deliberately put yourself in such a situation?
What do you think how much you can save if you invest the 30 hours of commuting threatening you in doing the work yourself? That and a few compromises on the wishes for a new build and a house also brings one within reach at your current place of residence or nearby.
And don’t forget the travel costs. Neither driving nor taking the train is getting cheaper.

PS:
Do your parents live near your current residence?
Then you can forget about someone quickly dropping by...