I need a Return Pump Suggestion: COR20 Not Enough

[EuroDriver]

I'm not getting enough flow out of one of my COR20's so I'm looking for suggestions for a more adequate pump, or how to solve my flow issue.

I have a 180 with a 43 gallon sump, dual returns, JBJ 1/3 hp chiller, IceCap 40w UV sterilizer and a manifold with 3 1" gate valves.

I'm running 2 COR20's - one on each return.
One is doing fine, pushing 420 gph under the following conditions:
Running speed: 50%
Min speed: 16%
Max speed: 75%
It runs 1" plumbing like this:
pump -> 1" pipe -> 1" elbow -> UV sterilizer -> Apex 1" flow meter - > 1" elbow - > barb -> 1" hose -> 1" elbow -> 1" return (a few unions, ball valves and a gate valve in between)

The second COR20 maxes out at 280 gph under these conditions:
Running speed: 99%
Min speed: 75%
Max speed: 100%
It's plumbed as follows:
pump -> 1" pipe -> 1" elbow -> 3/4" reducer -> elbow -> ball valve -> barb -> 3/4" hose -> TLF ball valve -> chiller -> 2 elbows -> 1" manifold -> 1" Apex flow meter -> 1" return

Although it seems that the chiller has a lot more piping, it's a pretty short run from the pump, as it's on the same side of the tank. Also, I'm stuck to reducing to 3/4" as that's the largest that goes in/out of the chiller. I need to get the flow up to 400 gph as that's the minimum the chiller calls for.

I wonder if I'm getting a false reading from the flow meter, since I initially installed it backwards and ran it for almost a month before I plugged it into the FMM and realized the mistake. However, the flow from the return nozzle visually does seem lower than the UV return.

What do you suggest? If this calls for a stronger pump, which do you suggest? I'd prefer to stick with a DC controllable pump.

TIA