Hello,
I am not in favor of the construct with the [Bausparvertrag].
With interest rates currently so low, a fixed rate over x years is better. This means repaying the [KFW loan] in parallel with the bank loan. After 10 years, everything is renegotiated anyway, and then you can see how the interest rates look.
Assuming something unforeseeable happens during saving with the [Bausparvertrag] and you cannot pay the installments for years. The [Bausparvertrag] will only be mature for delivery in 10 years if the installments have been paid. That could become a problem. Furthermore, the overall effective interest rate of all components is usually worse than just [KFW] and bank loan.