Hmmn...
Chapter's made me a bit unsure with regards to the direction Oda's going to go with Sanji tbh.
It is evident that Sanji has kingly traits and this is something even Gaban who has only just met Sanji can see but the "become a King, if you truly want to make Luffy the Pirate King" line feels more like a fork in the road as opposed to a one way street to me personally.
Sanji at his very core despises royalty and kings and doesn't want to stand over others. This is a self-conscious decision on his part that is very evident here.
Moreover, it is clear as day that even if it were for Luffy's sake, if Sanji is opposed to becoming a King, Luffy wouldn't want him to become one simply for his sake.
So I guess whether Sanji truly decides to become a King or rejects that route and chooses a completely different path will depend on how flexible he can be with regards to how he processes what a King is.
Sanji himself doesn't seem to be opposed to having Conquerors' haki and seems to be interested in it even, but it'll be interesting to see whether his will to fulfil that requirement outweighs said blocker in his mind, which imo there's nothing wrong with him having in the first place.
Maybe rather than developing the will to become a King per se, it's about accepting that he indeed does possess Kingly qualities and accepting the burden that comes with it, and that having it doesn't necessarily mean he will become another Judge and doesn't have to confine himself to the stereotype of what a King usually is.
So "Crownless King" still feels like the most fitting direction for Sanji.