My opinion differs from the other to some extent.
You have several factors going on.
First is how fast the Herbie overflow can drain water. I would agree the @aga31 about flow rates, "a 1" drain should be able to handle about 400-600 gph in gravity fed, and up to 900 gph in full siphon." This means that if you pump water into the display tank faster then it can be drained, you will have the tank overfloe or the return area of the sump run dry, no matter how big they are.
Second, is the flow rate of the return pump. The Jebao 8000 pump can pump opver 2000 GPH, although that will be reguced somewhat by the plimbing and the height you need to pump the water.
Third is the size of the return pump chamber. As you increase the pump flow rate, it's going to slightly raise the level of water in the main tank. To a point, this additional height of water allows the over flow to remove water ar a faster rate, un to the max the over flow can handle. It your return area is too small, you'll run it dry even before you have much flow cominig through the overflow.
I see a couple of problems in your system.
The pump's flow rate is beyond what the herbie overflow can deliver. As you can see the your system works fine at a low flow rate.
The return pump section is too small. I suspect this is your problem. Remove that far left set of baffles, and this will give you more water to work with, since you will now have more water you can add to the main display tank before the overflow needs to drain it at the same rate.
PS - You have a DC controllable pump. It was incorrect advice for someone to advice you to add a valve on the output side of the DC controllable pump, and use that to throttle back the flow. This will only put more stress on the pump, and you don't need it, because you can adjust the pump speed directly.
