Ideally, you should have the buoyancy at any point exactly equal the load on that point, then you would not need any rigidity or strength in the platform.
In the real world this is not possible because of changing loading as people or whatever move across the platform, wind loading, waves, etc. But if you keep the principle and ideal in mind, you can probably improve any real design, including reducing its strength requirements. The shallow water border area, probably meant primarily as a breakwater, either is neutrally buoyant or if contributing to the flotation of the center platform will require substantial reinforcement (especially at the border between it and the thicker part of the platform).
It would probably be better to make an independent ring shaped breakwater that is not rigidly connected to the float.
The overall shape of the platform is not important if you can keep the loading and buoyancy closely linked, and a more lozenge shaped (very enlongated hexagon) would probably be better for an anchored seastead – less current stress on the anchorage.