What is the total volume of water that could drain back/siphon into the sump? It might be part of the volume of the overflow after the weir plus the volume from the drop in level in the main tank behind the weir. The critical volume in the settlement chamber should be bigger.
I like to have big sumps, one third to a half the volume of the main tank. That way you can get lots of medium between the baffles, and still have a sizeable settlement chamber.
Settlement chamber on medium tank:
The overflow drain is on the other side by the wall, so you cannot see it. But I drilled out both sides, so you can see the level of the drain.
This has an internal pump.
Settlement chamber on large tank:
External pumps (main and backup). Overflow drain with loop.
Drip into small tank:
Much heavier drip into large tank:
I try to tune the drip to take about seven to ten days to drip in the total volume of the main tank and sump together.