You probably can't really escape the taxman's clutches anyway. It is mostly a matter of whom you pay, not if. Taxes exist to cover the costs of governing the people, providing infrastructure and services for the citizens and so on. Even if you can get unplugged from existing governments, someone has to pay for the infrastructure of the seastead and essential services. You can give it a different name, but charging your people to help pay these costs is basically the same thing as taxes. Not charging your people to help pay these costs would mean eventually sinking to the bottom of the ocean. These costs are likely to be relatively high, higher than what it takes to maintain roads and sewage and the like on land.

Unless a seastead were the size of Manhattan its citizens would have to forgo the cultural life, the parks and the wide choice of shopping and restaurants offered by large cities.

They might have to forego a lot of that anyway. I briefly considered applying for a job in Hawaii and I asked around regarding things like food in Hawaii (because I have special dietary needs). I was told basically that spam is a staple food, good luck getting meat and milk on a consistent/reliable basis as it sometimes isn't very available, things get flown in from far away so a lot of it is frozen not fresh, and what is available is quite pricey. There is plenty of sea food available, but I am allergic to shell fish and don't tolerate large quantities of fish well. In fact, I rarely eat fish.

A few years ago, I stayed in Port Aransas, Texas for a few days. I loved the place and have toyed with the idea of going back (as in living there). But I struggled to feed myself while there. There is a great deal of sea food and some very expensive restaurants and some real dives and a very limited grocery store. Given my dietary restrictions, these details made it a daily challenge to try to eat properly for my needs and I don't feel it really did an adequate job.

So if an island a short drive off the coast of Texas has challenges in offering "a wide choice of restaurants" and Hawaii has challenges with making sure food (in terms of variety and affordability) is consistently available, what on earth makes them think a floating platform will do better merely based on population size?