Yes, the amount is crazy, but I would like to point out that it is driven by the property.
Honestly, I find the amount normal to cheap for metropolitan areas like Frankfurt, for example! Normally, prices are seven-figure for detached single-family houses. Especially since the OP certainly will not build spartaneously.
I have nothing to do with construction financing professionally. Banking is a wide field, and just because someone is a "banker" doesn't mean they have experience in construction financing.
Some people just cannot imagine that there are bankers who have nothing to do with construction financing or loans in general, just like there are police officers who don't chase criminals on the street or shoot, doctors who don't work on patients, or pharmacists who don't work in a pharmacy. Unbelievable.
You might as well sleep under the bridge; private insolvency won't save you from that... and secondly, with a loan of that amount, you can expect a six-figure early repayment penalty.
First of all, he will not go into private insolvency upon sale. And secondly, I would like to see how a six-figure early repayment penalty would be calculated on a loan of 720k at the current low interest rate level? You do realize that the early repayment penalty roughly corresponds to lost profits and banks can only realize modest margins at the current interest rate level? Seven-figure means at least nearly 15% of the loan amount. If those were the times today, banks would be covering their buildings with gold leaf.
CONCLUSION: From my point of view, objectively considered, the OP’s situation is in the dark green area; he just has to personally decide whether he really wants the house.