Carrot has been a non-character for almost 4 years now.
Yamato was introduced in the middle of ACT 3 and her fate is intertwined with Momo's.
Neither of them is joining. You people are blinded by horniness, powerscalling and headcanon.
Luffy has stated multiple times that he's doing this for Tama.
She has nothing that holds her to Wano, no family, in fact, she's been featured with the strawhats for over 100 chapters now and has interacted more with Big Mom than with Wano characters.
Also, when you think about Wano's connections with Thriller Bark is becomes clear that Tama is a parallel to Brook.
She's the the first character introduced in the arc
She has an unfulfilled promise that Luffy can fulfill
Old/Young member new world inversion
shows up later in the arc to save two strawhats