This is cool! You're one of the few people I'm glad to have followed here.. Always posting interesting material!
I came across a post yesterday by @michaelmatthews called How To Build Freedom Island. While the article has a definite philosophical overtone, there's some interesting concepts in regards to hexagonal platforms and their architectural strengths when used in a modular fashion.
For seasteading, I think the most realistic and affordable approach is simply to buy up and then repurpose aftermarket cruise ships. They have already paid for themselves carrying tourists, so they can cost a lot less than building a custom floating structure, and they already have everything onboard that a seastead would need. Add some wind turbines, solar panels, replace the fuel tank with a fresh water tank and off you go.