Building a house from a distance. New construction 300 km away.

Distance doesn't make it easier. Build with a GU and, as often suggested here, find someone locally who regularly shows up on the construction site. Either way, you will never quite get what you would if you were on site. WhatsApp is on the decline among larger craftsmen due to the GDPR. But there are plenty of alternatives.

Is the OP still alive? Has he said anything yet?

Standing on the highway, waiting for ADAC, was on the way to the construction site. Bad luck.