The water level in the overflow box looks too high, a lower standpipe might help. It's hard to troubleshoot these type of problems from pics. Maybe wider slots in the overflow. Anyways, you want to bring the tank water level down somehow, and have as much room as possible in the sump. Check valves can fail, then you still have the worry of an overflow. Lockline going from the bulkhead to within an inch of the surface would do the trick too.
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