The ball valve on the main down pipe should be moved lower down. As low as possible. The outlet part on that pipe should be just under water. Else the water in the pipe cannot force the air bubbles out. Also need to securely fasten the main pipe to the cabinet. At least the main, the other 2 you can fasten or let it be. The principle behind this design is that this pipe is full of water with no bubbles inside. So the pipe will be a lot heavier than the other 2 pipes. Having it so high and the pipe will take a lot longer to get the bubbles out after restarting the return pump.
The secondary outlet pipe should be much deeper in the sump. This helps the main pipe to force the bubbles out. Ballvalve should be set so that less than 20% water goes down the secondary. Here the water should run down the sides of the pipe making no noise instead of free falling. 20% and less flow and the water would stay on the sides.
The emergency should end above the water line in the sump. Especially in your design where you cannot see into the box from the front of the tank. Rather let it make a noise down below, alerting you to the fact that something is wrong.
Did you use those bulkheads that you find at Builders warehouse or Chamberlains. If yes, you will see that their design is not great for our purpose. The bulkhead goes narrower inside then open up again. On main outlet not an issue as the complete pipe should be full of water. On the secondary it mess up with the idea of getting the water to cling to the insides of the pipe, free falling from the bulkhead down to your bend much lower down. This will create a noise nullifying your design. Take a piece of that white netting you used in the sump and roll it up. And push it down the secondary. The roll should be long enough to go down all the way to the bend. This will provide the water a path to cling on to instead of free falling. No need to do this on the other pipes.
I got the exact same overflow design. Check out from post 67, I had all these issues. And I had to redo the pipes at the end.
RiaanP Mermaid tank