New tank design graphic ... Tell me what you think...

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Your design is somewhat similiar what I've done.

I have a 20g refugium sitting to the side and slightly above my 75g sump. I run two drains (Dursos) with one split between the sump and the ref, with the flow directed to the ref via a valve. The ref flows to the sump from a 1" horizontal bulkhead to the sump on the skimmer side of the baffles.

Comments from mine
-I'm flowing ~1000gph split between between 2-2" drains. They're quiet but nothing like a system I heard (Skimmin's) where he had ~2000gph with 2 Herbies (1.5" mains, 1" buckups) Silent, plus there was no gurgling in the sump. I would suggest plan your display for a Herbie and the loudest thing in your system will be the pumps.
-refugium feed, I'm splitting a drain, doesn't take flow away from going back to the tank and what a lot of this is about is maxing flow in the display. A negative is you'll collect a bit a detritus but at least for me it's really not a problem.
-refugium drain, I was surprised how little mine flows. Always see 600 gph for a 1" but turned sideways and without any head I'm safe at ~300gph. The backup is a good idea. Do see a little scum on the ref surface so glad I've got a overflow for surface skimming.
-refugium isolation, with a second valve then just throwing in a heater and a HOB filter (and maybe moving LR to sump) it's a QT tank. Figure if something like ick could treat with hypo, then before putting it back on-line would just run fallow.