Financing offer with a concrete cost plan

No, as described, the repayment always includes KFW. So an 800 euro installment with 170 € for KfW and the rest for the "main loan":

I would set KfW to 10 years ... more does not make sense because the fixed interest period only runs for 10 years anyway. After that, with the 10-year loan, it has the advantage that I can include the not fully repaid KfW in the follow-up financing.

If the term is longer than 10 years, after the KfW loan expires, I would have to do a refinancing/follow-up financing only for the KfW and then again 5 years later for the "main loan". That doesn’t look good and the conditions would certainly be worse.

If I calculate the 15-year loan and the 10-year loan both with an 840 installment to make it comparable, then after 10 years I have already repaid 66k at the shorter term and 1.6%, while with the 15-year loan I have only repaid 52k. Roughly calculated .... I just repay 0.65% more monthly for 10 years, which already makes a big difference.

BTW: You can currently still get 1.6% or more on daily money accounts, if you’re lucky. The thing with saving up instead of special repayment was once explained to me by some economist, I don’t know what’s behind it ... I can ask the banker about it again.

I understand that the financing will run for longer than 30 years ... but there will be at least 1500€ per year available for special repayments. But since we are also planning to have children and I first want to get used to the monthly burdens, I chose this rate because it’s very safe for us.

BTW: No, you can't (with a risk-free investment without conditions).

No, as described, the repayment always includes KfW
So an 800 euro rate and of that 170 € for KfW and the rest for the "main loan":

I would set KfW for 10 years ... more doesn’t make sense, since the fixed interest period only runs for 10 years anyway.
Afterwards, with the 10-year loan, it would have the advantage that I can transfer the not fully repaid KfW to the follow-up financing.

With more than 10 years term, I would have to do a refinancing / follow-up financing only for the KfW after the KfW loan expires and 5 years later again for the "main loan". That’s not good and the conditions would surely be worse.

It may be that I misunderstood you now:
You want to fully repay the KfW within the first 10 years?
You will not achieve this with 170 euros per month.

Quick calculation:
50,000 euros with 1.25% interest = approx. 625 euros p.a. interest expense in the first year
170 euros * 12 months = 2,040 euros rate p.a.
2,040 euros - 625 euros = 1,415 euros
1,415 euros * 10 years = 14,150 repayment plus compound interest effect.
Everything else you would have to, if you don’t have additional funds free in 10 years, extend.
My KfW has a slightly higher interest rate, but in the constellation 10 / 1 / 20 (10 years fixed interest / 1 year interest-only / 20 years term) a repayment of approx. 4.3% (if I remember correctly – the 4 before the comma is right).
You would have a repayment share of 2.83%. That means you probably calculated the variant 10 / 3 / 30 – so for a 30-year term.

You would need a repayment of roughly 400 euros per month + interest to have fully repaid the KfW after 10 years.

No, I do not want to fully repay the KfW, I want to pay the minimum rate every month (just under €170) and refinance after 10 years since the fixed interest period will expire then.

Btw: I have a [Bestandstagesgeld], which is credited monthly at 1.75% interest ...and all that without any strings attached ;)

What is a fixed-term overnight deposit?

An overnight deposit is a product with a variable interest rate, which is adjusted regularly.

There may be periods with a guaranteed interest rate, but even the interest ends at some point.

So I don’t think this interest will last long and will then be adjusted.

Because why should a bank offer you this interest? It has no permanent reason to do so.