Construction company halts work despite overpayment

Tell me... What would you do differently from today’s perspective?
You have now been able to/gotten to gather quite a bit of experience. Some people can then draw their lessons from that.

To be honest, I don’t know if I would go through it again. At times I was mentally exhausted and we still aren’t completely through it.
The costs, if it had gone wrong, are insane. €20,000 for our court and lawyer fees, plus the expert witness at around €3,000, the opposing lawyer, and the bank’s commitment interest for the undrawn loan. But I also don’t know how the other route (accepting the demands of the builder) would have gone. How far would he have gone? What more would we have had to pay? Because of the expert witness, whom we ordered due to the lawsuit, we now know the defects of the shell construction, which would not have been the case otherwise. To recall, it started with him wanting additional costs from us due to static requirements, even though we had agreed on a guaranteed fixed lump sum price, which were based on the fact that instead of drywall walls in the attic, solid walls had to be installed to support the roof structure. That had absolutely nothing to do with the client’s wishes. His first offer was about €1,000, which we accepted. Two days later, an email came saying he had miscalculated and forgotten the concrete beam and that the whole thing would now cost about €6,000.
Since there were also defects in the execution—for example, the basement protrudes 10 cm too far out of the earth, etc.—that was the point where we intervened. In the end, he then stopped building. But how do you make the right decision? The risk of being stuck with all the costs is very high. The insolvency of the builder is also a big danger; then you’re left with everything. My decision back then was right for us. If we had had a performance bond, as legally required for contracts from January 2018, I would have viewed it a bit more relaxed. All builders should make sure to get this bond amounting to 5% of the contract sum and not let the builder dissuade them with any promises. And be careful when choosing the construction company—we bought the house from a seller and didn’t even know the executing construction company. In hindsight, it turned out that the executing construction company had one employee, who was the managing director. All work is subcontracted. I would never commission such a setup again.

I think it's great that you persevered and stayed "stubborn"!

My zodiac sign is Taurus, so it couldn't be any other way ;).

You somehow packed your answer into my quote. But in the end, it’s simple:

Such a forum is always a melting pot of all kinds of people. There are always people who will be difficult. But such people can also be good, for example, when someone wants to build their villa with a net income of 2,000 euros and then nobody says, "It will be fine." I mean, these are all just ordinary people who signed up at some point.

And as unsentimental as this sounds -> In the end, mistakes happened on your part. That the comments here were stupid. Right! That you share your experiences -> important. A forum lives from stories like these being told to the end.

Then let's hope that I can post a photo of our ready-to-move-in semi-detached house at the end of March.
If you write, mistakes have happened on my part, what could/should I have done differently?

Well, what would have happened if your legal dispute had not gone in your favor? From a financial perspective, your plan probably has a few risks (since I am not an expert, I am not assessing that!). My feeling is that your plan had relatively few safety nets. Is that a mistake? Yes! And you have already written yourself what you would do differently. But mistakes are part of life!