A few thoughts...
As far as I am concerned, OpenWorld is Open World. Everyone should have the 'chance' to participate in port battles.
I think that Capital ports should be considered invulnerable.
In order to limit any nation achieving total domination, a value has to be used for the 'over reach' of a nation. So as they over reach themselves their ports become more vulnerable. Imagine the RN with say 25 ports as a starting point, then they end up with 100, perhaps their ports invulnerability should be divided by 4, or maybe less as they will have more money and trade to pay for their occupation. Maybe the invulnerability of nation’s ports could be linked to the number of ships/players in the nation, though some governing handicap factor should be employed to prevent world domination by any one nation. So a diminishing scale so that the difference between 1000 players and 10k players is next to nothing or inversed to give the 1000 an advantage. For example we could say that a massive nation could well become carelessly over confident, and a smaller nation would have to be more careful and cunning, hence the advantage to a smaller nation.
So do we establish a status quo of port occupation and allow that to change only by a certain percentage.... perhaps we could have seasonal and political cycles that influence the status quo in favor of one or other nation(s).
One thing is certain and that is that not everyone will be happy with whatever we end up with, but it must be enjoyable for most everyone to take part in.