Delays with the general contractor, no deadline set according to § 650 Building Code

But had they accomplished anything yet?

Started with the gutting and then realized they had miscalculated (wanted to significantly increase a flat rate). Since then, back to individual trades - it was once an experiment.

And you didn't even pay for the started service? Then you were lucky or it wasn't worth the effort for the GU.

In mid-March, a sampling protocol with a price list even arrived. We found nearly 30 errors in it. Among other things, tiles were completely missing, sanitary objects entirely, heat pump completely, ventilation was suddenly no longer present in the residential basement contrary to the contract, etc. Since our correction list, it has been completely silent. For 3 weeks. No information on why nothing is progressing.

That really sets off alarm bells for me... how can such fundamental things be forgotten in such quantities? And then pretend nothing happened? I wouldn’t have waited 3 weeks for a call and would seriously consider the advice here regarding checking for a cancellation right etc.

It would be like that with goods. But even there, if the goods have been used so much that they can no longer be considered new, you would have to accept a reduction in the refund amount.

I think what you mean here points towards warranty. It is logical that with a used item, you cannot invoke the right of withdrawal. Also, this only applies 14 days after the purchase is concluded, provided you were informed about your right of withdrawal.
In the case of withdrawal under a construction contract, there are also the futile expenses of the builder, which here would be the sample selection and possibly even the building application. If the house was planned by the general contractor and now can no longer be implemented as such. But money is, as is well known, one thing; the lost time is another. Basically, you’re starting practically from scratch again now.

whether the sampling performance can already be considered properly provided

For that, the sampling must first be agreed upon as a service at all. If only material and quality are specified in the construction service description, the contractor can theoretically choose the decor himself. The fact that there must be sampling at all is by no means a given.

And you haven't even paid for the started service?

The work has already begun. However, there is also a criminal complaint against the general contractor. But that's another matter...