First off don't lower the full siphon pipe yet (primary, one with gate valve). This won't increase head pressure or flow, this is set by the gate valve and nothing else. Lowering the height will only drain more water to your sump and that's a pretty big overflow you got there. You may need to lower it if it's creating a vortex and sucking in air but I don't think that's an issue.
Seems to me the issue is your back up or emergency drain is too low. Try this procedure.
- Stick a coupling on there or swap out the pipe with a taller one. Start with something too tall that extends an inch or so above the top of the tank, you can cut it to the right size after the next step.
- Tighten the gate valve a little to raise the water level in the overflow, it's too low if the weir is dropping down too much causing waterfall noise. Water level in the overflow can be around 1/2" to 1/4" below tank water level, but set it as low as possible provided the noise is gone. The overflow should be completely silent now, if not check if the primary is creating a vortex and sucking in air, if it is the primary pipe needs to be shortened.
- Now with the overflow water level set, mark the back up drain pipe 1/2" above the overflow water level and cut it there. I recall you have three drain pipes, so consider running two back ups at this height, it'll help for next step.
- Now test the back up, with pump operating at desired or max output shut the gate valve completely but be ready to open it if needed. If the back up drain(s) can handle the flow without the tank overflowing you're good. If not you have to shorten them until they can, 1/4" at a time but note if you have to cut it more than once you'll need to lower your overflow water level in step 2.