Couple things I noticed...
In your first pic (external overflow box), why is the water level so low? I’d ditch the durso setup on your full siphon drain in favor of a low profile strainer. Then, use that gate valve to raise water level so a little water flows through your secondary drain. If the system was unplugged for the photo, disregard HAHA!
As far as your pump/sump dilemma, there probably isn’t any issue with your pump needing to be throttled back. From what I can tell, the actual pump section of your sump is super small. If possible, maybe reconfigure the baffles to give you some more volume there. That, or consider an ATO. If all else, add a downward facing 90° fitting to the pump’s intake to prevent it from sucking air when water level gets low.