Construction costs are currently skyrocketing

He refers to the past, when after the 10-year fixed term of his loans, he always had an advantage by simply "letting it run."

Well... in principle he's right, when it comes to the comparison with external banks. They, meaning the currently financing bank, of course make you an offer roughly 0.2-0.5% cheaper than another bank. That is a service advantage because they know you and the property. New banks have to check you and inspect the house... those are personnel costs.
However, the offer will always be adjusted to the time when the contract is to be concluded.
So if the average in 2024 is 6%, then your bank offers you 5.8 or so.
If you now conclude 2 years forward, then at today's interest rates plus 0.25% per year surcharge for the forward.
I cannot imagine that anything will change to your advantage in the medium term. However, I am a layman here, others are better informed.

Last week, Madame Lagarde announced further hikes of 50 points. In this respect, interest rates will continue to rise.

In the end, it is important that one can afford the future repayments. Striving to minimize losses by crystal ball gazing is, in my opinion, not advisable for long-term investments. Why not diversify and cover part through forwards and maybe a smaller part in the MSCI World via ETF and possibly special repayments in a proper fixed deposit? After all, you do not usually invest your funds entirely in ONE asset class. So why here?

I have a question regarding financing methods. I have googled a bit but couldn't find anything on it, maybe I'm just missing the right terms.
We don't want children and have been wondering if there is a method to make building significantly cheaper or even finance it at all under the current situation. The question is whether there is a method where you give up the house after our death? So it is not inherited.
(We don't see renting as an alternative. And yes, we really don't want children.)

A paid-off house is for us just a lot of unused capital as soon as we die.

I was once told that there were cheap plots of land that would pass into the ownership of the church after 100 years. Something along those lines is what I had in mind.

I was once told that there were cheap plots of land that would pass into the ownership of the church after 100 years. That's the kind of thing I had in mind.

Maybe you meant hereditary lease? That is sometimes offered by cities or the like.

Back to the church

You mean leasehold... you simply pay rent for, for example, 100 years and are not the owner of the land
That used to be quite normal here with us, but our bank, for example, said right away they don’t do that

You then buy a house on credit but don’t own the land beneath it
I mean, there were even problems (not for you) if the lease expires and is not renewed, then the heirs can demand the demolition