Are the financial resources really sufficient??

I have not read all the posts and therefore do not know if the special repayment of 5000€ is made consistently. If so, then you probably have a calculation error in there.

You forget that the annuity loan also wants to be repaid.

Example with your assumed 250,000 €:

Annuity loan: 200,000€
Nominal interest rate: 2.41%
Repayment: 2%

annual special repayment assumed: 5,000 €

Term: 20 years

results in: monthly burden of 735.00 €.
Remaining debt after 20 years: 26,280.74 €.

12x735€=8820 rate/year
Special repayment = 5000€/year

that makes a total payment of 276,400€ in 20 years minus your calculated interest of 52,680€ equals 223,720€ principal repayment, although you only took out 200k€. According to my calculation, you would already be done after about 18 years with approx. 47k€ interest costs.

The KFW loan could be repaid with 265.00 € at a maximum monthly burden of 1,000 €.

I calculated with 1.25% KFW as well as a repayment of 5% with a term of 10 years and a loan volume of 50,000 €:

monthly rate: 260.42€
Remaining debt after 10 years: 23,386.05 €.

KFW follow-up financing:
I assume the following here: nominal interest rate of 4.5%, repayment 7.5%.. although the bank will have a say in this.

Monthly repayment rate: 233.86 €. Remaining debt after 10 years: 1,286.51 €.

That fits.

If I now compare a pure annuity loan it looks as follows:

Annuity: 250,000 €
Nominal interest rate: 2.41 %
Repayment: 3.55 %
without special repayment!
monthly rate: 993.33 €
Remaining debt after 20 years: 17,778.28 €.

12x993.33€=11,919.96 rate/year
Special repayment = 5000€/year

that makes a total payment of 338,400€ in 20 years minus your calculated interest of 56,178€ equals 282,222€ principal repayment, although you only took out 250k€. According to my calculation, you would already be done after about 18.5 years with approx. 60.6k€ interest costs.

Mh... I still have to recalculate later. Although I've already double-checked the numbers, the devil is always in the details. But you're aware that in the example there is a remaining debt after 20 years, right? I assumed a yearly payment for the special repayment. Reality usually looks different.

You are right! I must have mistyped when entering it into the system. No idea what else could have gone wrong there. ops:

More and more numbers...

But that still can't be quite right.
As a comparison:
My offer at €235,000 (interest: 2.37% / repayment: 2.98% / 25 years) results in a rate of €1,041.
The offer created here at €250,000 (interest: 2.41% / repayment: 3.55% / 25 years) is supposed to result in a rate of €993.33 and be finished even sooner than my offer??
Am I making a mistake in reasoning or is something getting mixed up here??

... or is a lot getting mixed up here??

Yes.

2.41% interest + 3.55% repayment = 5.96%

5.96% of 250 K€ = €14,900

Divided by 12 months, this results in a monthly annuity of ~€1,240.

The remaining debt of the loan made here (250,000€/ interest: 2.41%/ installment: 993.33€/ 25 years) amounts to 48,060.58€ according to the online calculator, and the interest paid is 96,059.58€. This should now be correct and reasonable. Or??