I still think Yamato will join, and I don't think that Oda has changed his mind or anything like that, I mean Jinbei has just joined in Wano arc, and it doesn't make sense to make two characters join in the same arc, if that happened, Yamato would have stolen the lights from Jinbei.
even though Oda wasn't planning to make Yamato join at the end of Wano, he was still giving us hints that Yamato will join, and one of those hints was in chapter 1056 when Momo called the straw hats in their joining order and then called Yamato, the fact that this hint was one chapter before the end of Wano, means that Oda didn't change his mind, because it would be too late for him to do so, also if Oda was planning to make Yamato join in chapter 1057, why would he give us a hint in 1056, he didn't need to make that hint if she's just going to join in the next chapter, right?, Oda's hints usually refers to many chapters later.
finally I think there is an important reason behind Yamato's staying in Wano, not just powerscaling issues, since the writer can easily control the power levels, however it's really hard to control the flow of the story, because it needs planning, so Yamato had to stay for now because of the flow of the story, it'll probably be something related to Pluton.