clarification on I-Beams

I'd start by deciding how many lifting points and how the load will be supported on the platform. Check if the load is evenly distributed or if there are areas of more localized loads, because you'll need to reinforce there.

So, start design by placing beams connecting the lifting points and the most loaded areas. Proceed this way ending up with a frame of beams. Dimension beams individually or in the assembly as you'd do anyway.

Need an outter rim connecting them? Are you thinking about curved beams to fit the round shape? I'd think twice about that. Straight and square things are much easier to do. And can be assembled from off the shelf material.

You mentioned your assembly is round, but, do your platform needs to be round? Maybe a square one with liftings in the corners could do just fine.

I agree I beams would do great, at least with the mentioned initial approach.