Unfortunately, that's how it is. Nobody thinks about it beforehand. But don't let that stop you from designing the underfloor heating for 30°C installation planning (if calculated (!) possible) and simply not guaranteeing the temperature. Even with the 30°C installation planning, you will be able to reach 35°C. But then you only discuss the meters of pipe and the installation distances, not the temperatures.
He did say in the further discussion that he plans the installation distances at 10cm and theoretically, we could run well below the 35°C flow temperature because of that. However, I can't see how that would fit with the planning. I will probably only see how the system is ultimately designed once it is installed; I will not receive planning in advance and I can't take that out as a separate trade either, unfortunately, he won't agree to that.
There is so much in house construction that you should read up on in more detail, but where do you draw the line when you have contracted a general contractor? In the end, the realization often comes: "I could have also contracted individual trades." But especially the ignorance about the processes in construction and the associated fear of doing something wrong, forgetting, missing something, etc., drive many into the arms of a general contractor. Once the construction really progresses and you start reading into the subject matter with the trades in parallel, doubts often arise – of course, too late.
But I still stand by my decision; I really have no choice left – think positive! :)