"I was smart enough to use 16" OC framing on the initial build so going to composite shouldn't be an issue excepting for laying out a diagonal pattern."
You might want to check on that. I built mine with 14 inch spacing which I'm sure is what Trex recommended at that time. Also a diagonal pattern would require closer spacing, the angle means that the deck planks are spanning a longer distance.
I don't mean to be negative but it wouldn't be nice to have any sagging.
I wish you well.