Gas pedal and parking brake of the heater: Explanation correct?

The question is whether it has always been like this...

Then the pump is simply too weak for your system size, if you need more flow.

You don't need to use formal address, in forums people usually use informal address.
You can lower the maximum temperature, but the heating should never exceed 50 degrees if it was properly designed.
You probably received documents from the heating engineer about, for example, heating circuit lengths, heating load calculation, and the supply temperature the underfloor heating was designed for.

I will take a look at the documents. A question has spontaneously come up. During the day, I kept checking what the flow temperature was doing. Is it normal if it varies? Sometimes it was 24 degrees, sometimes 28 degrees, sometimes 30 degrees. Or should it actually always have a fixed value?

By the way, I have now set the heating curve to 0.2. It feels like the temperature has dropped a bit, although we still reach a good 23 degrees.

Yes, the supply temperature fluctuates, among other things, depending on the outside temperature (heating curve) and whether heating is currently active. Most systems do heat somewhat beyond the supply temperature and only start again when it drops below a certain value to prevent constant switching on and off. These are usually values in the range of 4 to 7 degrees.

It can take 24 hours or more before you notice changes to a floor heating system.

Does anyone know why I only get a maximum of 1l flow in the sight glasses? They are turned fully up

Didn't you have a modulating heat pump? It also reduces the flow when maximum power is not required. The flow is modulated down to achieve the optimal temperature difference in the heating circuit for the optimal operating point at the corresponding outside temperature. Theoretically, you can set the control of the heat pump in the Luxtronic from automatic to manual 100%. Then you can see what your hydraulics deliver. But for "daily use" that makes no sense because you then have an efficiency loss. Is the password for the settings known?