Quite possible because I fixed the interest rates completely.
Sounds like you don't actually know that. Sounds like Sparkasse and building society constructions. But actually even they should provide you with this basic information.
if the loan-to-value is higher, better conditions can be offered
It's not like you don't have any equity left, right?
But I think it's because of my Schufa. In my younger years everything was on credit and many credit cards, plus a low salary.
Most banks in the real estate sector offer conditions that are independent of the Schufa; it’s only used to decide whether to lend you money or not. The conditions are generally based solely on the loan-to-value ratio.
How long should I fix the interest rates in my case? Maximum 10 years and then renegotiate? What do you think?
For God's sake, no. You should deal with this topic more thoroughly.
Definitely get comparison offers from other banks, or also contact one or two financial brokers. Actually, with the property data and the available equity, a 30-year term should come out at around 2.x%.
In plain language, that means you could be done significantly earlier. At 2.9% you would be finished in 27 years with the same rate!!!