I’ve daydreamed about this sort of thing a lot, and my conclusions are generally the same as his. I think he complicates matters, though, with his focus on the idea of sovereignty. I would argue that in reality, there’s no such thing. Any group of people who decide to call themselves an independent nation are independent exactly to the degree to which they are able to defend themselves.
Trying to make yourself inoffensive, so that you don’t need to defend yourself, is a non-starter. No one contemplating seasteading has the intention of living in poverty, and anyone enjoying a western standard of living would be a target.
You can try to get friends to defend you, but you need to find someone who is able and willing. Most countries that are able (America for example), aren’t likely to exert themselves to protect people they see as tax dodgers. (The unique quality of Sealand was that it was clearly under the umbrella of the UK, but had a peculiar legal status that made it de facto independent.)
So there’s really two avenues to starting your own nation. First, get a strong power to accept your “sovereignty” and also be friendly enough that they will defend you. To pull that off, you generally need to be a persecuted minority. Second, hire yourself an army.
Where I differ from Borgen is I don’t see the latter as impossible. To be sure, you’d need to be seriously rich, but there are people in the world who can afford to retain a decent mercenary force. You don’t need one that can fight world powers; just one that can deter pirates and whatever nearby dictators might cast a covetous eye your way.
What happens next? That’s where the daydream begins.