Thanks. Ive had the jebao 12000 in my amazon cart by coincidence.Your full-siphon overflows will be submerged a bit into the water. You will control this by putting valves on the lines that go down into your sump and then closing them off until the flow from the pump matches the flow from the overflows, so they run full-siphon and quiet. This will take some tuning to achieve and is a reason why I highly recommend using the Jebao DCP pumps because they have a very fine adjustability on their controller that can be used to match the valves rather than trying to match the valves to the pumps. Then, the emergency overflow will be a bit higher than the full-siphon overflow but not so high that it will let the tank overflow. The emergency overflow should remain fully open. It’s not uncommon to get a little bit of a trickle going down the emergency drain either. If I were you, I’d recommend making the two 1” bulkheads your full-siphon overflows and then making your 1.5” the emergency drain.
Also, you need to make sure that your sump can handle the extra water volume if you lose power and the main tank drains down to where the full-siphon drains are. This should be easy for you since you have a 150 gallon tank with a 75 gallon sump. I’ve managed it just fine with my 220 that has a 55 gallon sump.
What if i went with an emergency drain 1" right above the overflow i already drilled? Id really like the main drain in the center of the tank.