I'd close in the ends too and put a door in one end so you can get in there to work on wiring. To my way of thinking you didn't sink the posts far enough into the ground, but you have a lot of them. A typical pole building, which will withstand winds no conventional stud type building on a slab will take, has green treat columns on 9 foot centers sunk into the ground 4 feet with a 80 lb bag of bag crete poured around each one at the bottom.
I think you have to look at it from the standpoint of a building. But instead of shingles you got a solar panel roof on your building, and you only got a one way slope on the roof. The wind load on the solar panel side can be high so you build inverted roof trusses to support that load. You just need some treated 2 x 4 and plate nails to truss those 2 x 6's so they don't bend under load. And you put in cross bracing in the building to strengthen it end to end and side to side.
Your span there is only 12 feet? Drive around the countryside and find a farm someplace that has a pole frame building, stop in there and ask the farmer if you can look at how it's built. You'll see right away what you have to do. Pole frame buildings are incredibly strong because they're built like a bridge or lattice wind turbine tower - they will take 150 mph winds that will rip the steel off them, but the frame survives. We have pole frame buildings here on our farm that have 60 foot clear span with 2 x 6 trusses on 9 foot centers. But they're what is called a "W" type truss. Our buildings here get 30+ tons of snow load on that 60 foot clear span in the winter and they don't give because the truss transfers the load to the columns. And inverted W-type half truss is really easy to build with some 2 x 4's and plate nails.
Screw corrugated building steel to it on three sides, which is only about 30 bucks per 3' x 9' sheet, and it automatically braces the building sideways too.
Sure, you can throw big massive type beams at the problem. I believe in using engineering instead of mass.
I would still consider those Unirac rails to mount your solar panels on the roof of your pole building. They're designed for the job and they're not all that expensive considering the size of your array and the amount of panels you have to mount.
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Chris