Realize house construction financing or let it fail?

You have summarized that very well! We could indeed imagine a fresh start, but of course we are kicking ourselves for having wasted two years senselessly.

You have not wasted the two years, you have learned from them. And if the perfect house for you now results from this, that is great. And as far as the cost increases in construction are concerned, I know at least regarding wood that prices at the woodworking fairs in the USA have already begun to decline again.

Since almost every craftsman in Rhön serves construction sites in Munich and Frankfurt, some in Switzerland, they don’t give away their work here. Cheaper, yes certainly, but not dirt cheap.
Already in 2017, 2000 euros per sqm was a good benchmark.
That was a long time ago and currently everything is running out of control anyway.
We are similar with the OP.

Without a plan, no advice can be given. Presumably, it comes down to abandoning the current plan and redesigning it incorporating the basement as living space.
The OP must simply not take out 50cm in the middle to reduce the sqm.

Nevertheless, and with that I return to the original question: Are 600 TEUR with 5.3 TEUR net not a bit too high?

My assessment as a non-expert: No.

Nevertheless, and with that I return to the initial question: Aren't 600 TEUR with 5.3 TEUR net a bit too high?

It cannot be answered in general, because all other expenses are unknown. (Or were they listed somewhere?) For example, one or even two "gas guzzlers" that cost a lot or company car drivers for free? 1% interest + 3% repayment results in a 2000 € installment! At 30% we end up at 1600 €!

Would you maybe tell us where your construction project is planned?

I ask because I read somewhere in the thread [Franken-Nord-Ost]. Sure, 200sqm + basement is not cheap, but nearly €800k for the house without land seems too much for the region.

All of this based on one(!) offer sounds more like a defensive offer to me.

It cannot be answered across the board because all other expenses are unknown. (Or were they listed somewhere?) For example, one or even two "gas guzzlers" that cost a lot or company car drivers for free? 1% interest + 3% repayment results in a €2000 installment! At 30% we end up at €1600!

We have set ourselves exactly the €2,000 monthly annuity rate regardless of the financing volume. Apart from one company car, there are no vehicles. The calculative debt service capacity might not even be the problem, but rather the size of the loan, maybe for the bank, certainly for my psyche.