Set the heating correctly

so at 8 degrees outside temperature I have a flow temperature of 27 degrees this results in a room temperature of at least 23 degrees (Bad EG) and a max temperature of about 24 degrees (Schlafzimmer) I think these are great values

why is it that baths always cool down faster than the other rooms?

The greater the temperature difference (inside/outside), the higher the transmission heat losses... simple physics

so the guest WC is set to 20 degrees like in the other rooms, then it should be because it is only 4 sqm small and half of the room has no underfloor heating installed due to the floor-to-ceiling shower, or the window is drafty, it will be one of the two

No, rather the combination of all the things you mentioned... and of course the physics

so at the moment I have a consumption of about 4-5 cubic meters in 24 hours including hot water at temperatures between -5 and 0 degrees, shock ventilating all rooms for about 10 minutes, that's not bad, right?