Insanely low costs with you. I tried it the other day at about 5-10 degrees outside temperature. Without basement about 15-18 kilowatt hours, with basement about 30 / day. And there I had the ERR downstairs only on level 2!! Although I don't know if there is a difference in terms of current whether level 2 or 6, because the flow temperatures for the upper floor are about 30 degrees anyway and for the basement only mixed...
I also think that your 30kWh has no significance. Especially if it is otherwise unheated, it takes quite a while to reach the temperature. The heat from the water goes into the building fabric and as a result, the return flow gets colder. It takes a while until the whole thing is warmed through; with us it was easily 1.5 days for the ground floor alone with increased consumption. You should reread the function of the ERR carefully. OB 2 or 6 is initially irrelevant for the water temperature. An ERR does nothing more than measure the temperature in the room and close the valves from the set value onwards so that no more warm water flows in. So until the temperature value for your set 2 is reached, the water flows with the same flow temperature as if you had set a 6. Possibly the value was not even reached by the end of the test. Overall, however, you shouldn't expect too much, 220sqm +120sqm basement is already a huge building, and even with a brine-water heat pump you will consume quite a bit. We were also at 500kWh in December for 160sqm KfW 55 city villa, and that with a good performance factor of 4.2.