Guys I'm so discouraged, P***** Off, ready to give up

[Aquattro]

I've got a slightly different take on this. The tank is young, but not new. It's settled, biologically, for the most part. The cloudiness also looks like clam spawn, but if you say it's green, ok, maybe algae.

If this was my tank, and I'll probably get some disagreement, I would leave it alone. I would take the filter off the back, add some carbon to a bag and toss it in the sump. With no measurable PO4, I wouldn't worry about GFO right now.
I would not be doing large water changes, and in fact, would do no water changes. If it's algae, let it burn itself out. Feed sparingly. Keep the tank aerated. Buy a real skimmer for in sump. Stop messing with it, there is nothing in your parameters that dings any alarms.
I would remove the glass tops, let the light in. Don't move the lights around, don't change their cycle. Make a pot of coffee, relax. It's not an emergency or something to quit over. It just needs TLC.
I assume the fish are fine? Not gasping for air at the surface? Then algae isn't consuming excess O2. Still not sure that's algae. Is it green? Or is the water just cloudy?
Sometimes the best thing to do is nothing. Often you can cause more problems than you solve by trying to fix everything overnight.