I’d second the gulper pump recommendation.
In my last boat I had a similar sump setup. I tried to repair, no joy. I bought an entire sump/pump/switch setup new, and still had issues. The duckbill valve and the size of the pump was an issue, even with a 100% new setup.
The manufacturer of the sump box setup even knew there was a problem with the design, I called for support and they ended up sending me a bigger pump as I experienced airlocks often. I also had to remove the duckbill valve. If I remember the brand of the setup I’ll post it.
The setup did work well after the bigger pump, and it worked well for over 1.5 years at that. But it was a pain to get right.
I knew I had plans to sell the boat in the future, (I just sold it) but if I had longer term plans I was surely going to invest the time and effort to redesign using the gulper style system.
A search here on TF will probably yield results on the gulper.