I don't ever understand valves on the drain ? Unless they are for complete shutoff during maintenance the outflow is completely dictated by the pump ..
I use DC pumps which are easily adjusted to only feed the tank so the drain only gets as much as it can handle .
I would try a larger pipe for the outflow also ? All my mega overflows have 1 1/2 out/ drain and 3/4- 1 inch return .
IMO ideally you want the outflow to easily flow through the pipe which a larger one would accommodate better ?