Rent or sell an old house and buy a condominium

in lifetime and quality of life... but everyone sees that differently... so you will probably end up with stocks/funds depending on which advisor you have.

and I imagine 900 euros cold plus additional costs, which are certainly higher than for a newer house (high electricity and heating costs).

Whether this is a moderate rent, we cannot judge because we do not know the rental index. But if you want to rent out a property in need of renovation or even damp or rather unkempt, then you have to steer the rent extremely downward. The rent reflects the value of the apartment or house. Your flowery description would not even convince the city to rent the apartment for socially disadvantaged people or immigrants/asylum seekers...

since it is uninsulated, i.e. among other things, for example, you are not allowed to place cupboards too close to the wall, you have to heat sufficiently otherwise there is a risk of mold, furthermore, it has to be renovated (wallpaper, floors) the heating system itself is new but here and there there are certain things you have to pay attention to.

I'll put it this way: either you invest so that you have an adequately rentable apartment or you sell the property.

then I looked for your question... it seems to be that you don't know what to do with all the money?
The answer is obvious:

We are currently building a house

But I am not the type for stocks and also not the type for renting out,

And how would you invest the money then

Special repayment!

Thank you for the detailed assessment. However, I realize that the situation is quite complex and depends on many different things, etc. Perhaps I simply did not present it thoroughly enough.