Giving virtually monopoly powers to the subsidized shipping industries of New York. Virtually all the packet shipping lines of the US operated from New York.
If a cargo was to be shipped from Boston to New Orleans, it had to be carried by an American flagged ship.
Making it utterly pointless for European ships to sail anywhere but New York or Boston or Philadelphia. It cost the same to do business in South Carolina as New York, but New York was 800 miles closer. That dynamic changes dramatically when taxes get reduced from 40-50% down to 13%. Suddenly there is a clear profit motive to sail to Norfolk or Charleston.
The reason why New York, Boston, or Philadelphia handled more ships is because they had the capacity of handle large numbers of ships.
Capitalization of the Southern ports would have remedied that, and with additional profits to be made by sailing to Southern ports instead of Northeastern ones, they would have done so, thereby depriving New York of most of it's traffic.
Show me the United States Congress authorized legislation that subsidized the New York shipping industry.
Packets operated out of New York because it was the best port on the Atlantic seaboard, with the larges capacity for shipping.
There was absolutely no reason a British ship would not call on say Charleston or Wilmington, if the cargo’s owner requested that as the debarkation point. There is nothing in the 1817 Navigation act that precluded any foreign ship from calling on any US port with a cargo originating from another country.
In the years before the war. Almost all harbor improvements to Southern ports was paid for by the United States Government. Your fantasy scenario of millions being poured into Southern port facilities is just that a fantasy.