Hi Raaden, I'm about a year behind you with a similar project in a colder climate. It's great to have pioneers like yourself go before me so I can learn at your expense.
The free exchange of information will certainly help us all. I'll try to share my experience while I pick your brain.
Have you experimented with the shadow cast by a 30" deep vat vs. a shallower vat? Are you planing to have a raised growing platform, or live rock base?
Will OSB stand up to humidity compared to regular fir plywood? From my experience (using it under commercial tanks), it tends to "flake off", fall apart, and bow. It also seems to be less tolerant to salt exposure. Up here in Canada, 3/4" fir plywood is about the same price as OSB ($27.00/sht).
I have used teak and mahogany (marine grade) plywood ($175.00/sht) for tank and vat construction, but have found (good one side) 3/4" fir plywood to be adequate and a world cheaper. The vats I have made have been between 12 - 24", so a eurobrace was all that was necessary (no external frame). The 12" deep vats required no bracing and had no significant bow.
With our cold climate up here, we can't use pressure treated wood, as ventilation is limited during winter months. Is cedar, cypress, or ponderosa pine an affordable option down there?