With 460kW of heated screed and a proper hydraulic balancing, 3 to 4kW minimum output is definitely not oversized. However, if the hydraulic balancing is done half-heartedly and too many ERRs restrict the flow during normal operation, the heating system will still suffer a bit.
Of course a heat load calculation was done, but it only refers to the coldest day.
That is normal. I still can't believe 18 kW. "Box" too big and probably half of the heating circuits throttled by ERR, then you have the described dilemma.