Long fixed interest period: BANK vs KFW

I actually still lack too many details about the installments, the actually planned special repayments, etc. At this point, you might as well roll the dice. The statement

Ca 1400 wäre auch unser Limit.

is also nonsensical if this is already exceeded with the 10-year fixed interest periods. I therefore assume that the max. 1400€ applies only to the bank loan.

I have now, just for fun, calculated with installments that I have set.

Option 1
Bank installment €1307.00 (initial repayment approx. 2.93%)
KFW installment €355.00 (initial repayment approx. 2.78%)
Total burden €1662
Option 2
Bank installment €1342.00 (initial repayment approx. 2.0%)
KFW installment €320.00 (initial repayment approx. 3.1%)
Total burden €1662

The installments for the KFW loan in both options were chosen so that the KFW loan is repaid after approx. 30 years.
Repayment-free years were disregarded.
The bank installment in option 2 was set with an initial repayment of 2%.
The bank installment in option 1 was determined by the total burden minus the KFW installment.
The comparison of the two outstanding debts (bank + KFW) is made after 20 years.
The interest rate for the loan extension after 10 years (with constant installments), at which both loans have identical residual debts after 20 years, is approx. 5%. If the interest rate is below 5%, option 1 is better; if the interest rate is above 5%, option 2 is better.

variant 2 would be for the risk-averse who can save a lot (or make special repayments), e.g. to pay off the kfw after 10 years and not require follow-up financing.

actually, the question of the fixed interest period is always just one of risk awareness. you have to be clear that a longer fixed interest period is expensive. even a few percentage points difference are huge differences.

10 years 1.45% with €1210 rate: €260,000 remaining debt, €100,000 paid off, €45,000 interest (end of loan)
10 years 2.5% with €1210 rate: €297,000 remaining debt, €63,000 paid off, €82,000 interest (loan continues)
20 years 2.5% with €1210 rate: €217,000 remaining debt, €143,000 paid off, €147,000 interest (end of loan)

consequently, you basically pay more interest for a longer fixed interest period, but do not really pay off the house in the end. after 15 years you would have paid off the same amount at 2.5% (100,000 repaid) as with the 10-year fixed interest at 1.45%. so you pay 5 years of "interest" for 10 years longer fixed interest period. (of course, you also pay off something during those additional 5 years) roughly speaking: double loan term for only about 1.5 times the repayment.

as musketier already wrote, no one can help you if you don’t provide the information. there are still some good ideas depending on the financing behavior. a popular option is also a split into 3 loans. 1x smaller part for 10 years, which is then paid off. a main part with a long fixed interest period (and you only pay the interest) and of course kfw. the small loan is paid off quickly, the kfw drags along (because of low interest) and special repayments are made on the big one.