Sorry for the really late reply! ><
There was someone from here replying to my posts in Reddit making taunts based on my opinion, so I've been avoiding touching certain subjects until the story gives a clearer hint on the outcome.
Ah… people should be more open-minded even if they disagree.
But, since you asked, I feel Yamato is being built up to become a samurai. Despite his desire to go to sea, it feels like there's both a bond forming with Momonosuke, who Yamato believes to be the bringer of the dawn, and some hints that deep down Yamato also desires to be a samurai (like in his quick flashback). I'm not closing the door on him joining, but I'm feeling the most likely outcome is that he decides to stay and help rebuild Wano with Momo, both being "guardian spirits" of the country. I can totally see he being named "Kozuki Yamato" and becoming a surrogate brother to Momo.
Oh, I see! That does make sense! I guess we will see! Though Yamato would be a weird samurai with her weapon that she currently using xD