I don't think it's cut and dried yet. Yamato wants to- at least, according to that poor translation- go on the ship, which is not the same thing as joining the crew. We also know Caribou wants to do the same thing after Wano, and we've seen that sailing with the Strawhats does not make you a Strawhat.
That means she is 99% likely to go with the Strawhats after Wano. That can be
-as an official Strawhat by the end of Wano
-in order to build her further up to be a Strawhat in the future
-simply as an ally.
The only thing I'm certain about with regards to Yamato is that Oda wants us to be discussing her as a potential Strawhat.
If she is going to be a Strawhat she could really do with getting a facial scar or something distinctive like that. She is far too similar facially to Nami for my liking- I can tell them apart, but c'mon Oda, you can do better than slightly thicker eyebrows and lower eyelashes.