She's next nakama not a side character.
Nah, I honestly don't see it.
Oda is not going to hide Yamato's true abilities for a fight with Kaido and just end it in a chapter or two.
Like Oda has:
- Refused to reveal Yamato's Zoan form let alone her Hybrid form
- Denied Yamato a proper fight even in Base.
That Yamato hasn't fought anyone else yet, has > precedence than your speculation on how the arc would unfold. Oda just isn't that kind of writer.
Jack was never defeated because we hadn't seen his Hybrid form yet.
The Dukes got to low diff Zoan Jack as hype for their Sulong.
The Scabbards lost to Kaido, but Oda gave them a good fight first.
Yamato is going to be fighting Kaido for 3+ chapters unless she faces someone else before the Kaido fight (e.g. Jack). That's just how Oda writes.