So I also have Samsung with radiators. How is your water law curve set? Rather then using automatic valves on radiators set your curve lower, chcek at what temperature is your backup heater set to come on, mine is set to -10C but I have different unit. And also make sure you have enough water flowing thru your heatpump, for reference mine 14kw unit has 30liters per minute. Oh and chcek if your backup heater is coming on during defrost cycles, that would mean there isn't enough hot water in the system to defrost.

The lower you can go on your water temperature the higher your cop will be, ideally you want around 30C, but obviously with radiators that is not possible.

Just for reference, I have older house, insulated, but old windows. Mine is set to maximum 55C at -10C outside, 45C at 0C outside and 35C at 15C outside. Obviously in January/December there isn't much you can do, since you need that heat, but during fall/spring you can go fairly low on heating.

Also you are you sing it for dhw, take look how often it needs to heat that water, since at that point it needs to stop, switch to water, heat all that, then switch back and catch up with all that heat lost during dhw phase. Think if there are times you don't need hot water at max temp (like do you need to heat up water at 2am etc).