The developer has let us down - what to do now?

And? Does that make the topic less interesting?

As long as no one comes along and says, hello, I have a similar problem in Fresh, there is probably some truth to that, one could say.

I'll bring up the topic again ...

We have now sent very many emails to the builder over a long period. No defect rectification. The first self-performed repair has now been reimbursed by the construction company, the second invoice has not been paid so far. A payment reminder has been sent.
Further defects have been reminded again (to really meet the deadlines).
Let's see what happens this time.

I would report that to a roaring reporter (newspaper or TV)

In our apartment, the bank had no problem with us receiving the withheld money ourselves by presenting invoices for defect repairs (e.g. completely new parquet flooring). Basically: withholding due to parquet damage -> developer does nothing -> 3 registered letters with return receipt, each with a 2-week deadline -> after the 3rd time "no response" or just empty talk -> we hired a company ourselves that removed and replaced the entire parquet, sent a registered letter and return receipt with a message stating "old parquet will be placed on the terrace for pickup on day X, if not collected, disposal will be ordered at the developer's expense". It is important that the "old material," if it exists, must not be disposed of. Everything MUST be very well documented beforehand, especially the damage must be verifiable through pictures and witnesses (several and independent companies, for example). This way, there would be no problem proving in court afterwards that the damage really existed.

In our case, loan repayment did not start as long as the loan was not fully disbursed – also interest was charged on the already disbursed money and costs on the money not yet disbursed. The fixed interest period shrinks and shrinks without repayment taking place. If possible – offset through special repayment.

In case of construction defects that are not immediately apparent, one relies on a procedure to secure evidence. Unfortunately, this does not mean that the construction expert makes an effort and correctly assesses the matters just because he was appointed by the court. Also, such a process currently takes up to several years.

Important: Legally, the client must always pay an invoice. Even if the developer is supposed to cover it. Emails are NOTHING in such matters, only registered letters with return receipt.

You do realize that the OP hasn't been online for half a year and probably won't respond.

Not only that, but in the original post the term "Deutsche Fertighaus Holding" is written a total of 9 times, even in places where it is completely redundant / unnecessary. This forum is very strong in search engines when it comes to house-related questions. The frequent mention of this term (also grammatically incorrect in sentence structure) only leads me to suspect that a competitor has paid some SEO guy here to create a bit of anti-advertising that can then be found as high as possible on Google.

Well.. this user had at least written a lot about all topics beforehand. Of course, it could also have been intentional to make it inconspicuous.