Then you either heat with unnecessarily high system temperatures, which is neither beneficial for the efficiency nor the operating behavior of the heat pump, or the towel radiator barely contributes to handling the heating load.
Heating with high system temperatures means strong throttling of the underfloor heating. This results in only a low volume flow in the heating circuit. Heat pumps generally operate with high volume flow. If the volume flow is too low, the heat pump will not be able to get rid of the heat and has to shut down. Even with a separating buffer like you have, this problem can only be healed slightly. Therefore, the recommendation is electric towel radiators or wall heating. The room temperatures are then not controlled via the ERR, which can only switch on and off, but via the flow rate at the heating circuit distributor. The ERR should ideally be kept open permanently in almost all rooms to guarantee sufficient volume flow. They can be used in the bedroom. It is best to deactivate the unused ERR so that no one can accidentally cut off the volume flow and you also save electricity. The room temperature is set by means of the flow of the heating circuits! This should be calculated in advance (hydraulic balancing). In the first years, optimization must be done (thermal balancing). A smart control system is not required for this, only a thermometer and some time.
The radiator does not contribute, it does not even get lukewarm...
And how does one deactivate the ERR? Because in the storage rooms I don’t need any heating at all (the window is always open there anyway) and usually not in the bedrooms either.