I believe you misunderstand the KFW loan a little bit.
The fixed interest period of the loan is a maximum of 10 years. This means that the current conditions are only valid for 10 years. After 10 years, you will have a certain remaining debt. You can either repay it, partially repay it, or you have to arrange a follow-up financing. So, there is no special termination right. KFW will make you a new offer shortly before the 10 years expire, which will be based on the market interest rates at that time, or you find another bank to take over the follow-up financing. The initially calculated term of the KFW loan only applies on the assumption that the interest rate would remain the same after the 10 years.