Variable loan possible / sensible?

... which bring him a total of €6000 extra per year. Not much but better than nothing. There is also a bigger project running, from which he collects 20-25K, ...

Collecting = before tax ≠ profit after tax

The biggest headache for me right now is the double burden of rent and land loan. If you build a house right away, this double burden is lower and much shorter.

Then there are three possibilities: - Prepayment too high - Loss from self-employment or business - High advertising costs in non-self-employed work. The tax office certainly does not just simply pay something back in case of profits. Or I have been doing something wrong my whole life.

I don't understand that.
You are supposed to pay a €1000 installment + €700 rent for 3 years now.
After the house construction, you are supposed to pay €1700 installment.
Where is the difference?

But otherwise, it all sounds totally understandable. In those 2-3 years, you will already notice what/how much is possible and how strong the desire for a house is.
The desire for homeownership just recently came up for you because of the nesting instinct. And maybe you want 2 children, then 80 sqm condominium is no longer necessarily a lot and maybe you would rather do without one more dress.
But only you know that.