Anyway regarding Yamato’s role, I think there are 2 possibilities: log keeper, or tank. Although it’s been stated that Nami is the one who’s been log keeping their voyage, it’s possible that Yamato has her own log besides of Sunny’s log. It’s like she has her own journal, like Oden has.
And if Yamato is the “tank”, she is more of a defensive instead of offensive like Zoro who is a combatant. Luffy Zoro Sanji Jinbe will most likely deal with the biggest threats, so Yamato fits if she’s there to protect the weaker ones. Luffy Zoro Sanji Jinbe are not always there to protect weaker ones most of time, therefore Yamato will be there to protect the ones who needs protection