Maintenance - Boats rust, even glasfiber boats need to be taken up once a year and painted etc..
We're not talking about a boat here, we're talking about platforms (around 50 to 100 meters wide) that can interlock and expand to a city sized seastead.
The popular consensus has been that it should be built out of concrete. This will last hundreds if not thousands of years. One of our experts has built similar structures and there are many working examples of floating concrete platforms (one even has a landing strip on it).
Okay so I like some of the ideas and I hadn't thought of locating on top of existing cables. The rest I knew about or had thought about myself.
However it all falls apart with your platform solution - its basically just another housing project in an existing city/harbor/bay under existing laws.
Are prices even going to be radically lower? Probably not by much - even factoring in the high city real estate prices much will be eaten by the platform costs.
Another architecturally edgy building for the rich - with luck maybe middle class city folk, but not really much "homesteading" or "sea" in it.
Floating concrete at sea won't work, it will crumble very fast. I know this because it has been done before at D-day WWII; they floated over temporary giant hollow concrete blocks and used them as floating landing harbors.
Concrete is very strong... and very brittle, it can't flex in the sea and so it breaks and crumbles.
You also can't just link many platforms, the links would be under immense stress and would either break or need maintenance.
Maybe you could build a static platform at some low depth ocean location maybe even get lucky and find such a location overlapping with cables and international trade - you "only" need some 1 billion dollars + luck to get that going I think
For real seasteading to work you would need some kind of ultra strong and flexible rustfree material that is also very cheap to build your floating areas from. Sign me up when hyper-diamond is invented.
EDIT: And again after spending all those millions you end with a relatively small area even if it works.
If you spent a similar amount of money on a small army and a plot of land you could have a REAL nation regardless of what local authorities wanted - with cheap ground that doesn't need maintenance and area to do a range of activities.
No matter what though starting a nation from scratch costs at least a billion or two with luck I would guess.