In One Piece, I Become the Overlord of the Seas from the Start

Any Novels with a Pirate Protagonist Who Loves Using a Sword?

My kid's run out of books to read.

Any One Piece Stories? No Saintly Protagonists, and the Longer the Better

One Piece.

There Are Damn Few Good One Piece Novels. Come Fight Me If You Disagree

One Piece Stories Where the Hero Is Invincible from the Start

Any Novels Where Someone Transmigrates Into One Piece as Luffy or Zoro?

Looking for something with a System.

One Piece Stories Where the Protagonist Has a System and a Devil Fruit

One Piece novels where the protagonist has a powerful Devil Fruit and a System, and is cheerful and outgoing.

A Protagonist with Conqueror's Haki

The more, the better.