Thanks Pearl and William. I think I may be stuck with rocks as I already moved 2 tonne onto the platform. There are some advantages to this design (very buoyant offset by very heavy), a persons weight barely impacts the structure by comparison. So it feels solid when you get on it. Also by using rocks, the weight to buoyancy ratio won't change much (other than plant matter) and roots can weave there way right into the crates and beyond, hopefully adding structural integrity. Sand unfortunately would fall through, unless I cover the platform in weed mat (or something) but then roots would struggle to work their way through. Sand above the water line would get heavier when it rains and potentially sink the platform beyond the desired amount and (I think) never dry out again. Is that right or have I confused myself?
The disadvantage of my approach is everything you mentioned and I'll add, if it breaks, it could be dangerous and a hell of a job to clean up. In hindsight, i should of explored lighter options but hopefully this will still be ok.
Re nutrients, I do plan on sending rainwater directly into the pool via a nearby roof. We are coming into a very dry summer here so won't expect much anytime soon. So I'll attempt to work with the water I have. Fingers crossed!