I went on site yesterday to evaluate again before proceeding with my plan, which was to add additional structure + use the existing. The existing wooden structure was to blame for the sign failing. I knew it wasn't perfect, but in the rush of doing this extremely last minute + the original provided mounting brackets not working, I let some things slide and it's my fault.
For future people, what failed was the twisting and sagging of the 4x4's. I honestly had a feeling this was a problem. I was talked out to adding some additional structure. It was a mistake. At minimum, several 2x4's should have been added to link the two cross beams, making it one solid structure. I don't know if this would have solved the problem, but it definitely would have helped.
So I'm throwing out everything existing. I'm using the cross members that were supplied and modifying them to they'll fit. I'm also cutting 3/8" off each end of the sign to get some tolerance so the sign will slide in and have some room for growth.
In addition, I'm adding a 2"x2"x.1875" aluminum angle to the bottom of the sign which will be supported by some 12"x6" steel corner brackets. Lastly, I'll be adding the same 2" angle aluminum to the edge of the sign (approximately 18") and then mounting a piece to the posts.
In total, 2 - 1.5"x1.5" cross beams for warping and cross winds.. 1 - 2"x2" Angle with brackets to take shear. Additional 2"x"2" angle to take a combination of sheet and cross winds.
My only question is: Can I get away without adding a backer board at this point? It's 2" thick, 20lbs DUNA HDU. For $200, I can add a piece of .063" Matte Black Aluminum. For $115 I can add a piece of 3mm ACM. Right now I don't have the days to use MDO as the painting and prep work will just take too much time.