Can we afford this?

Just under 4,000 and a payment of 1,200 is no longer possible?
That assessment really makes me think.

I only looked at what you currently have freely available from your current income. From your numbers, I came to €1,750, but not only the house payment has to be paid with that. And then your income should drop by €650 when the first child arrives. So €1,750 becomes €1,100. With an estimated payment of €1,200. Do you see the problem?
It may be that some costs are lower during parental leave (one doesn’t drive to work, i.e., reduced fuel costs, no canteen meals, etc.).

In my case, the ratios are similar (income to payment), and it works, but to be honest, my freely available budget is actually too low. After all, the car will eventually have to be replaced, reserves have to be built up, and I would actually like to make extra repayments... You can’t live hand to mouth!
In my case, I had planned differently, but then I spent more on the outdoor area, and that’s why the monthly burden is now higher than originally planned. Also, as of now, no child benefit for homebuyers, which I thought I was entitled to. And suddenly, a few hundred are missing every month, and it becomes less comfortable than expected...

You basically have the best conditions. Both civil servants... that’s gold dust!! Secure salary, increase foreseeable. You can definitely tackle this. It will just be quite challenging for a while once you start a family.
And as an East German, let me say... why should you work only 25% after two years? At least 50% should be doable... and then the calculation looks much friendlier!

We all have very different resources and lifestyles. Therefore, some people with an extended leisure budget can no longer imagine a disciplined household.

As I said, I only took your numbers. Whether you can restrict yourselves somewhere... you have to see for yourselves.

yes, go ahead.
However, I would first plan the children and then build the house.
The advantage is that if a) doesn't work out b) doesn't stand empty and remind you of a) every day.
If a) works out, you already have a more realistic view of the need and the available income as well as costs with the children for b).