Loan-to-value 80%, interest rate fixed for 15 years. Is that fair?

I am also currently in negotiations (15 years full repayment, 60% expiration) and the interest rate seems too good to be true to me as well. Even the big well-known brokers cannot offer me such a good interest rate, although the expiration is better. Is there KFW or something like that included?

No, that is not included.

Although I have to say that since the offer there have already been 2 increases from the bank. So currently "only" something like 1.5 would be possible.
And there is probably a great discount for the full repayment model, otherwise I think another 0.4 would go on top.