How much water should/must come from the main line?

Yes, that is probably the solution, but the installer is not available and I definitely will not attend to it myself.

So little pressure will also be interesting for the sprinklers. They assume 4 bar. We have a maximum of 6 bar. But I also suspect the valve.

Just take a look at the cross-sections of the main valve and the piping in general... and then the cross-section for the temporary valve, where the water flow is also disadvantageously guided around the corner. That can't work.

Do the test: open both valves fully, then slowly close the main valve of the waterworks, I estimate you can close it up to 75% without noticeably less coming out of the small valve, because the bottleneck is there anyway.

But then it was definitely the plumber who messed up, he knew the screed was coming, so why does he install such a valve and even say that the screed layers can connect to it.... I could really get upset about such unnecessary nonsense.

I also had a tap like that, which was more than enough for the small screed machine. However, I also have 5 bar in front of the pressure reducer here.

Unfortunately, I only have a 17t screed silo mixer, not a *small* screed machine.