We build in such a way that we handle everything ourselves. We select the people for the respective trades ourselves. Although we do not assist, we take care of everything.
Ok, then you have no choice but to settle it within the family. Running something like that through the insurance probably won't work since it is not a technical defect. Just because you don't like it doesn't count at all in this case, especially if you only have that one sheet as [„Angebot/Bauleistungsbeschreibung“].
Normally even more would have gone wrong with individual awarding on your side.
It's like ordering a car. You want a car, red with 4 wheels. Whether you get an Audi or a Fiat is written in the stars...
I often feel that you are a bit schadenfreude. Something can always go wrong, I know people who hired a good construction company and a good site manager, and still something went wrong.