So it's gonna be "I either have it correct or the writing is bad because I connected the pieces incorrectly"?
Yamato would certainly be an awkward inclusion to the SHs if it happens (the usual formula just isn't in place here), he is a last-minute idea from Oda after all. But I don't agree with the idea that his inclusion would be a cheat on the reader, or even a "disappointing conclusion" when all the evidence was (clumsily placed) right in front of our eyes all along..
No, it's about how the story presented itself. Nothing justifies spending so much time with Momonosuke and purposely avoiding any meeting and interaction with the Straw Hats just to do a 180 at the last minute.
And I see I'm not the only one that feels that way. A lot of people have manifested the same thoughts, even among those who think Yamato will join, as is the case with Daz.
There's something else to add as to why I don't think Yamato is joining. As the story progresses week to week, I set up a few checkpoints as flags that confirm or deny my expectations.
For instance, when Momonosuke and Luffy returned to Onigashima and we get a chapter ending with them and Yamato together against Kaido, I thought to myself: "Ok, this is a perfect crossroads. They will fight together, and we can see if Yamato gets closer to Luffy or Momonosuke. This is a good chance to see some chemistry between Yamato and his maybe future captain." And then Luffy sent Yamato off with Momo immediately. There were several others over the course of the story, and it kept checking on Yamato bonding with Momonosuke rather than the Straw Hats.
After all, I didn't come up with the "guardian spirit of Wano" thing. Oda did.
I didn't come up with Yamato wondering if he could become a samurai. Oda did.
And so on.
But one of those checkpoints I had set up was that at some moment Yamato would face the question: does Wano need to be guarded? And then there was this exchange that ends with Yamato realising something, but we never saw what conclusion he reached:
This feels like a pretty big moment that will be followed-up in the epilogue.
Another checkpoint that I'm waiting to see in the coming weeks is if/how Yamato will react once he sees Wano for the first time in 20 years. Because people insist he was trapped in Wano, but he was not, he was trapped in Onigashima. When he last saw Wano, the place was still mostly pristine, it was after Oden's death that Orochi sped up faction construction and degraded the environment. Yamato has never seen Wano in its current state, it's not the place he remembers. So, his reaction to that, and what kind of expressions and thoughts he will manifest (if any) will also be a good sign of his ultimate decision.