Every Mugi has been presented doing some stuff related to their rol, maybe Robin was the exception, but doesn't seems to be the case of Yamato, simply doesn't have nothing to do on the boat... This could change being Momo's protector or some stuff like that... But ... Now doesn't seems to be the case XD
We already have Yamao's role. People just don't like it because it means that she most likely won't join.
But yes, as you said in the case of basically every straw hat Oda hinted at their role early on.
Each member was either already professionally working in their position, had their skills alluded to or was frequently placed in that very position before they joined. It's just laying the groundwork for what's to come.
Again, people say we don't know Yamao's role, but we do. If you look at everything her character has done since she was introduced there's a very consistent theme running. Be it how 80% of her panel time was spent guarding Momo, or how she said she is willing to die to protect him and deeply regretted how she wasn't strong enough to do so before, or how her immutable df is literally known as the guardian deity of Wano.
Meanwhile Carrot's got the same kind of foreshadowing Jinbe had for his role as helmsman.
There's a reason why every single Carrot supporter has no doubt that if Carrot does join then her role will most certainly be the lookout/scout. Whereas Yamao fans are forced to work with roles like 28 year old cabin boy, chronicler (at the end of the story) or the "Oden" of the crew.