Which is......
Wrong.
Zoro is not even close to this narrative place.
In reality, there are no deuteragonists in One Piece since this is a largle multi characters story (and not just three characters like Anciant greek dramas)
In a regular stories (and One Piece is - overall - a regular story, not an anciant greek drama) the most important character after the protagonist, is the Antagonist.
The most important character after Luffy is therefore by narrative logic,
Blackbeard, NOT Zoro.