Construction financing... realistic and sensible?

It can make sense, but it doesn't have to. This depends on the interest rate level expected at the end of the fixed interest period, how high the remaining debt is, how much you still have left, and how uncomfortable you feel with this residual risk.

Alternatively, it can be sensible to choose a longer fixed interest period.

Does it then make sense to already secure the possible residual debt of the amortizing loan after the fixed interest period in a building savings contract?

IMHO no. The money that is supposed to be paid into the building savings contract should instead be directly used for the repayment of the amortizing loan. Security is obtained through a long fixed interest period.

Many banks only offer a fixed interest rate for 10 years and then want to sell expensive building savings contracts or even finance through an insurance policy and fix the rate there for 20 years. I personally have not yet found a bank that fixes the interest rate for 20 years on an annuity loan. Do you have any experience with this or can you even give a recommendation?

15 years should be possible with most banks. Longer fixed interest rates were offered by a few banks and mainly insurance companies about 2 years ago. I cannot say if this is still the case. To avoid having to check with every bank or insurance company, it is easiest to inquire with intermediaries, e.g. [Interhyp.p], [Dr. Kle.n] or similar.

I personally have not yet found a bank that locks in the interest rate for 20 years on an annuity loan.
Do you have any experience with this or can you even give a recommendation?

I could imagine that it has to do with the low interest rates. When we finalized our financing almost exactly two years ago, we had several offers from banks through a financing broker in advance, which guaranteed the interest rates for over twenty years. As a "bird in the hand" type, of course we took advantage of that.
The bankers here in the forum can certainly explain this.

Many banks only offer a fixed interest rate for 10 years and then want to sell expensive building savings contracts or even finance through an insurance company and fix it there for 20 years.
Personally, I have not yet found a bank that fixes the interest rate for 20 years on an annuity loan.
Do you have any experience with this or can even give a recommendation?

A colleague at work took out financing at the Sparkasse just over 3 months ago and got the interest rate fixed for 20 years. Of course, there are percentage surcharges the longer you want to secure the interest rate.

I myself never had a single bank that didn’t offer at least 15 years. Back then, the "interest-wise" strongest competitor was the Commerzbank. However, they did not offer a repayment change. But in the end, we also signed with the Sparkasse (not our house bank) because the offer was (hard for me to believe) unbeatable.
I would never have thought myself to inquire about financing with them because they were always more expensive than everyone else. An intermediary got us the offer.

Oh yes, and Hypovereinsbank and Deutsche Bank also offered 20 years. But that was not an option for us since we did not need longer than 15 years.