House construction still realistic despite rising interest rates / construction costs?

That's exactly how you can and should arrange it. But don't make a tenancy agreement out of it in the notarial deed; instead, pay a usage compensation (then the rules of a tenancy agreement do not apply to you). Handing over in a broom-clean condition is also appropriate – as a new tenant or owner, you want to carry out your own ideas anyway.

P.S. I've done it like this twice in the last 10 years.

Thank you very much, that helps us a lot. Is the usage compensation then about the amount of a local customary rent?

Yes, we also sold our house and had a right of use for another 12 months during which our new building was completed. Our notary did not cooperate free of charge, because this is essentially an increase in the sales proceeds, which is not quantified. So for a fee, which in turn was not allowed to be called "rent," otherwise tenancy law would apply. So a currently popular practice, but one that is on thin ice (but worked perfectly for us).

We recorded exactly that in the purchase contract as well. Usage compensation with an extra clause stating that no tenancy is created. Nevertheless, the move-out must be announced 3 months in advance. We found that fair for both parties. The notary also had no comments here with us.

Which clinker did you want? :)

Sorry for OT: We had our eye on the aquaterra Feuerberg, DF from the ABC group. Regards

Demanding the nonsensical, KFW40+ AND mandatory district heating for the allocation of plots has also upset us. Unfortunately, according to the purchase contract and land register entry, we are not even allowed to install other hot water or heat-generating systems

I know the situation with district heating here in Wolfsburg as well. However, in the development plans, only fossil energy sources (oil, wood, gas) are prohibited. If I understand correctly, renewable energies may not be prohibited.
Therefore, you can see heat pumps and solar roofs everywhere in the new development areas.
Only geothermal energy is actively prevented because everything here is a water or spring protection area. Thus, only surface collectors are possible, if at all.

With the framework data, I would calmly choose a 15-year fixed interest rate and put the saved interest into faster repayment.
Honestly, what is supposed to happen in 15 years that a M.Eng. and a master optician can no longer afford the follow-up loan?

In my opinion, the interest rates of 2.5-3% are currently far from something you would want to "lock in" for 25 years. We saw a similar, but not as pronounced, jump at the beginning of 2015... it went quickly from 1.6% to 2.4% (15-year term)... and then it went down again, to the low of the years 20/21... 1%...