How to fix saggy fiberglass roof around AC unit?

picture looks fine to me. Can't really comment on the questions except to ask why are you trying to reinvent the wheel with the way the support beams are configured? It seems that typically a sagging AC is because there was a water leak that compromised the roof structure.
Do be mindful of added weight. In other words don't replace existing 2 x 2 lumber with 2 x 3 or 2 x 4 because it is easier to get or "stronger"