You need to control pump over the intake.
Ball valves on the return line not the intake.
This re enforces the overflow being #1.
If it doesn't get to the pump it won't mess with overflow.
Simple explanation? what works? 3000 gph
input and 4000gph out put ( overflow box not maxed out)
OR 4000 gph
input with 3000 GPH
No matter what pump or flow of it(being the pump) the "overflow box" has to be able to handle "all of it".
The ball valves can be left open (forever) on the intake(which will never see more water then the pump sends it), but they ,or one is better placed in the return position IMO
I run digitally controlled pumps on almost all my sumps now(almost all$$).