Maximum loan amount based on our monthly burden

Is this a joke?

Although it's off-topic, YES!

I am grateful that my wife mostly cushions it. Sometimes I think it's due to female genes. I couldn't imagine raising 2 children (I'm talking up to 7 years old) alone. My nerves wouldn't handle it.

Opinions do differ somewhat. In my opinion, one aspect is always somewhat overlooked: for the money, you do get something in return (the house with the plot) – and given how building land and real estate prices have developed over the last 10 years, at least the value of the land will not decrease.
By building a house (and paying off the loan), you are basically creating added value.
Of course, the finances have to add up overall. And I know how much many people like to plan everything. I am neither in favor of naive approaches nor of detailed financial planning for the next 50 years. Based on the situation described by the OP, I believe that financing around €400,000 is definitely doable.
(Off-Topic: I have been following the development of building land prices in the Palatinate since 2004. If good infrastructure is available – shopping facilities, doctors, schools, etc. – then prices have increased extremely. It is developing roughly like 10 years earlier in Switzerland – and even today there is no sign of a price decrease for building land there. By this I mean: if I can buy building land and realize a house construction today, then waiting makes no sense!).

That is all well and good, but why massively lower the standard of living now to potentially benefit from a speculative added value in the distant future?

I share the same view regarding the development of land prices.

The consideration or concern (a bit exaggerated) I have is rather that the house I will build in 8 months will not have the demand value (if interest rates rise again, e.g. to 4-5%, and thus house building costs drop again due to decreased demand) in x years.

In other words, if for some reason I want to sell my house, no one will want to pay me the actual construction price because the same house can be built for €100k less.

That doesn’t bother me as long as I live in it and see it as a retirement provision.

Or is this consideration wrong?

Many thanks for your assessments. A loan of up to 400,000 euros would be acceptable from your point of view.

The costs for a child, which you mentioned several times, I have taken into account in the above calculation, but only with just over 600 euros per month as an average value.

Yesterday I was at a real estate agent, who told me that we could also get a loan amount of up to 500,000 euros at an effective annual interest rate of 1.97% and a 2.5% repayment rate, which would correspond to a monthly payment of 1,800 euros, with a 5% prepayment option and free change of the repayment rate. The remaining debt after 20 years without prepayment would be 188,000 euros. To me, that sounds plausible in itself.
Do you perhaps also have an opinion on the agent’s statement? I am not interested in getting into debt at all costs.

That's all well and good, but why massively lower the standard of living now in order to potentially benefit from a speculative added value in the distant future?

No one has talked about massively lowering the standard of living. The standard of living varies greatly from person to person – what's important is that building the house could maintain the existing standard of living.