Current financing offer from the house bank

And that is exactly what I did not do and also do not consider it sensible at all. With my remaining debt, there is little to fear. For the follow-up financing, an increase in installments is also considered; depending on how much I repay beyond the building savings contract and how the interest rate develops exactly, I will probably become a full repayer in about 11-12 years with a €2000 installment. My goal is definitely not to pay off a house over 30 years, which I consider extremely unreasonable, also with regard to the maintenance costs that will then arise.
If others plan for 35 years and then fix the rate for only 10 years, that is their problem. There is definitely not only one solution, "as long a fixed interest rate as possible." Depending on the financing and personal risk tolerance, other variants are also rational.

For me, it looks something like this:
2% for follow-up financing -> bet fully paid off, the fools who paid 1.7% on everything back then...
2.5% -> still good
3% -> doesn’t matter, both about equally good
4% -> I just need one more year than with 3%, but no apocalypse
5% and higher -> then the debts are probably inflated away
You can play all that through calmly today. And in this case, there is a decent upside with limited downside. If at the end of the 2020s a major crisis manifests and interest rates plummet again, I will do a forward and take it with me. If at the end of the 2020s a strong interest rate increase should be indicated, there is also the possibility of a forward. So I definitely have options for action. In addition, there is a solid ETF savings plan running; if absolutely necessary, some of that can still be redirected into financing. But I would not now strictly plan the stock market into financing, only always as an option in case conditions are favorable.

I will take out a building savings contract with minimal contribution and secure an interest rate of 1.0% after 20 years (18 years of repayment).

I will conclude a home savings contract with minimal payment and secure an interest rate of 1.0% after 20 years (18 years repayment included)

With a 30-year fixed interest rate, I look after 10 years how the interest rates/loan-to-value ratios stand: either I refinance at better conditions or I enjoy the comparatively low (old) interest rate then. A 0.5% difference over 10 years is a good and affordable risk premium if I thereby possibly buy 20 years of lower interest rates in times of rising rates!

It's nonsense to buy pseudo-security instead of putting the money directly into repayment. By the way, it's not 0.5% but for me 0.7% or double the interest. Tends not to get cheaper anymore, as you have explained in detail.

Congratulations. You have just completely refuted the entire concept. First, you pay a 1-1.25% upfront fee. Then you invest your money at 0.25% or so for many years instead of using it to repay your first loan. This is called a negative interest spread transaction. And the "minimum payment" on your outstanding balance will probably be around €1000 or more per month so that you have a building savings contract ready for allocation. And lastly, I haven’t heard of a 1% loan interest rate with the building saver, more like 2-2.5%. At first, you tell us how great it is to put your saved installment into stocks for returns and that prices around Stuttgart can’t fall anyway. Now you come up with a disguised TA... It's nonsense to buy pseudo-security instead of putting the money directly into repayment. By the way, it's not 0.5% but for me 0.7%, or double the interest. It’s not likely to get cheaper, as you have explained in detail. The only thing topping the senselessness of this construction is a TA.