I had a similar problem on one of my setups. I resolves it with a Y splitter. If you installed a Y split (probably with a pressure valve but I don't like to use them because it can strain the pump restricting it's flow) and feed back half the water to the sump it might help out. This way It doesn't apply any pressure to the pump by restricting the flow and half the water just gets double filtered through the sump and it will reduce the amount being fed to the tank giving the overflows some time to catch up.
Here is where I would install it:
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