I faced the same problem: floor seemed too flexible, poor gelcoating, spider cracks. What to do.......
I put down a layer of fiberglass
mat and resin to strengthen the floor, pretty much over the area you see that is slightly different in color. Sanding down the surface was a HORRIBLE job: I had to make like a pretzel to get it done and I hope never to have to do that again.. The idea was to paint
the surface (Interlux Brightside, good stuff) so that recoating would be quick and easy.
It worked: the floor is much stronger. The grate I had from before, made out of teak, with the cross-slats contoured on a drum sander to fit the floor. The advantage of teak is that once it is made, water can't hurt it. The disadvantage is that teak dulls planer blades quickly. Should really be worked with carbide tools.