Floor heating setting - desired temperature

I asked myself whether it might make more sense to control not only by [Vorlauf] but also by [Rücklauf]. My controller could do that. The heat demand of the house can theoretically be recognized by the [Rücklauftemperatur]. Does anyone have experience with this?

The energy demand is calculated from the supply line, return line, and volume flow as far as I know. Heat pumps are probably controlled by the return line.

However, my knowledge ends there.

As an example:
Flow 30°, return 25° (everything ok)
Flow 30°, return 28° (return too warm, could have flow reduced)
Flow 30°, return 22° (return too cold, could have flow increased)

Flow temperature 30°, return temperature 28° (return temperature too warm, could have the flow temperature lowered) this has now happened. Gradually lowered the heating curve to 0.3, unfortunately the bathroom is no longer warm enough. So it needs to be a bit higher again.

unfortunately the bath is no longer warm enough now.

Is the valve for the bath in the heating circuit distributor fully open now?

Is the valve of the bathroom in the heating circuit distributor now fully open?

Yes, it is fully open, there is a double casement window in the bathroom and at the moment it is really very cold tonight -7 degrees.