flow sensor with activation condition for return pump control

It is a very good solution to control the flow of the return pump with a flow sensor, but in the practice of this solution, I have encountered a problem. I deployed the flow sensor at the end of the pipe where the overflow is connected to the sump. I want to keep the flow rate of the overflow constant to keep the water level of the overflow constant. However, the flow rate measured by the flow sensor is lagging and unstable before the overflow tank reaches the siphon. Only after the siphon siphon is reached can the flow be relatively stable. Therefore, I hope that the activation of the flow sensor can be controlled by time and water level. When the pump is started, it is guaranteed to run at full speed. When a certain water level is reached, the flow sensor is activated to control the flow to the desired value. Thanks GHL