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Well, you have to look at it in a nuanced way. On the one hand, 30k is not an amount that should give you headaches, and you will get a follow-up offer from KfW. The interest rates can't rise so much that the interest on a 30k debt hurts you.

So you share the same opinion as HilfeHilfe, better the fully amortizing loan over 25 years + a part via KfW instead of the "large" annuity loan over 25 years with residual debt. That is relatively clear.

The term stated for the KfW loan was 25 years. Can't that be set to 20 years and only the fully amortizing loan to 25 years?

The higher the repayment, the higher the installment and the shorter the term. I believe you can also conclude KfW as a full repayment borrower for 10 years. The advantage is that you do not allow yourself to be tempted to misuse the accrued special repayment for other purposes.