The problem with Yamato joining is that she has no concrete goal that she can only achieve by travelling around the world
All other Straw Hats have such a goal
Luffy: Needs to travel to find the final island and become Pirate King
Zoro: Needs to go out and challenge the world's best sword fighters to become the best
Nami: Needs to circumvent the globe to create a map of the entire world
Usopp: Needs to go out to sea to become the strongest warrior (admittedly that's vaguer than the other goals)
Chopper: Need to go out and learn about medicine from all kinds of places to be able to cure all illnesses
Robin: Needs to go to all islands containing Poneglyphs to understand the history of the world
and then Vivi: With Cobra potentially dead, she needs the SHs to help her either get back to Alabasta or wherever else she needs to go now
Franky: Has created the Sunny and wants his Magnum Opus to circumvent the world and prove the that his ship is the best
Brook: Needs to go back to the Reverse Mountain to reunite with Laboon
Jinbei: Wants to improve human-fishmen relations and thus has to travel the world and make connections (I admit this one is a little vague as well)
Meanwhile Yamato: Wants to go out to sea because Oden did so as well, she wants to experience the same kind of adventure he did
If she wants to larp, have her go to Wano Anime Expo, but not on board of the Thousand Sunny