So it was time to test it a few days ago. One of the first things you'll find researching the use of a Rubbermaid stock tanks for sump use is that the POS bulkhead they sell them with should be replaced with a real one or some day you'll have a leak. From all I found, 1.25" Sch80 was the one to replace it with. Well, it's a bit bigger than some molded flanges on the inside of the container, so I had to grind them down.
I tried hard and suspected I didn't get them 100% flat (not sticking up, not too much gone, they needed to be flat). I start filling up it up, bulkhead if fully submersed, no leaks. Good. Filled it up another 6" and it starts making a puddle on the floor. Tried to tighten it more, though I know too tight would have been bad ... still didn't help.
Draining the tank below the height of my magdrive, as the bulkhead is nearly on the floor of the tank, was a PITA. Did some research and flipped it around so the gasket was on the outside of the tank - should have worked since the outside as 100% flat, but once again, a puddle on the floor after it seemed to be holding. Did some more research and then did what so many people say not to do.
I had some RTV on hand from an automotive project, so I put a ring around the gasket on the inside of the tank and also put some on the outside just for kicks. Hand-tight and let it sit for ~1.5 days. No leaks.


Fill up the overflow - no leaks. Filled up the sump and tank to let the entire system run, no leaks!
The overflow is working too good right now. Props out to lilscoots for recommending to use the 1.5" for a full syphon and not the 2". Right now I'm still awaiting gate valves to arrive. During start-up, the 2" durso barely gets going before the full syphon comes on hard and drains the overflow. At that point it sounds like a stuck toilet and the water pouring into the overflow makes you think you're at a splash park. I'm periodically running the system still and eagerly awaiting when I can properly regulate the overflow internal height.

I made a return manifold too. Right now all outputs are 1.5" and only the above water portions are glued tight. The rest I can still adjust. Each T has a slip 45, so I have a wide arrangement of rotation to direct the water flow in the tank. I was going for max flow - might try restricting them to a smaller opening to get greater current once I see how the fish poop settles in there.

For the sump, I still plan to build a 2-piece foam cover to help conserve heat and reduce evaporation. The mechanical media portion has it's own cover, so I shouldn't need to remove the main cover to clean the mechanical media:

I'm stillt trying to figure out what I want to do for more lighting. The rock work I'm also not sure on. It doesn't go up as high as I might like, but I also don't want thousands of pounds of rock or any pressure on the rear pane. I'm also still awaiting new algae magnet pads to clean the front pane. Here are a couple full tank shots anyways (I just realized it's only room lighting, no LED in photos:

