If your wife is covered by statutory health insurance, then later have the children insured through her. The photovoltaics for children may not be that high, but a contribution-free co-insurance has its benefits. Besides, statutory health insurers "take care" more than the [Beihilfe] or the photovoltaics. Our daughter is therefore insured through my wife.
A long fixed interest period is definitely recommended. You are still young, and the salary still makes some jumps thanks to the age levels. But what about the place of work? Is it secure? As a civil servant, you can basically be transferred at any time, within your federal state or, as a federal civil servant, theoretically nationwide. Quote from a senior official when it came to office closures: "Buildings can be sold, marriages divorced. We assist with both."
And if you renovate, then do it properly. Everything that creates dirt or will be needed in the next 10 years should be done right away. It is always easier before moving in. Afterwards, it becomes difficult and usually ends in a compromise.