With a system that large, you would need to rethink your methods and expectations compared to an inside tank. You're in the realm of saltwater farming and need to look at how it's done there.
I've seen it done, but only where the region's temperature aligns with a reef - Hawaii, Florida, etc...
Minnesota would not be a good option, for example.
Where weather is extreme, you'll need to consider what farmers do: greenhouses. Basically, you'll need to create a structure to house the pond while making use of the sunlight.
Rain is not a big deal, even when exposed. You will need to monitor and adjust if needed, but in that size, it'll work.
Predation is also not a big deal overall. I called some saltwater farmers in southern TX and they didn't know what I was concerned about. Their response was - it may happen, but they're not going to swarm the pond. Having enough hiding places should be fine.
Of the ones I've seen, the real threat is making a giant warm yellow water pool of green hair algae in a slow stagnant saltwater pond - yuck.
Huge storms of dirt, animal activity (feces, etc...), dirty rain, insects, etc... Think of a team of polluting experts out there.
Flow and gas exchange- you will need to move a lot of water constantly to make this work. You will need cliffs and waterfalls to create anywhere near the surface action you need.
Your power bill will be massive.
Export - your skimmers would need to be small buildings that are built on site - think aquaculture farmers.
I considered this, but it was out of my $$$ range to do it right. I was hoping nature would foot part of the bill, but that's only true if the ocean is a pipe away.
If I were to consider it again, I'd make an internal pool/greenhouse room to make the project less painful.