My Yamato opinion remains the same: I see a high likelihood of him joining, but can’t say I would consider the road to said outcome smooth.
I personally don't think the clues or setup is strong enough for Yamato joining, but I think you're right about it being possibly rocky if Oda did go in this direction. I mean just look at Jinbeis journey to becoming part of the crew. If anything it tells us that Oda isn't afraid to branch out from the cast used on many of the early members.
Yamatos “Case” has a lot of factors going for it, but a lot of these are just so…blunt. Stuff like the connection to Ace, the appearance on volume 100, stating up front that he wants to leave and so on are all valid “clues” for Yamato joining, but they’re also wildly different compared to stuff like Frankys whole Sea Train escapade with Sanji and Usopp or Brook putting off a veritable showcase of personality in front of the entire crew.
Having a connection to Ace is a good point, but its also shared with Otama which is why I don't think it holds a much weight. My guess is that Oda wants to showcase Yamato's strength, personality and leadership skills in the effort to make them a viable candidate for supervising Momonosuke while the country rebuilds under the rule of the former Shogunate.
And yes… in most scenarios Oda wouldn't bother putting this much effort into characters that will lose plot relevance once the crew leave Wano; but my theory is that if Momonosuke is indeed Uranus, then he will indeed be important to the final war, which would mean he needs to not only develop his power, but also become a man (which has already been achieved through his aging). Yamatos guidance is therefore crucial towards him (Momo) being relevant.
Ultimately, I can’t help but see it as Oda deploying a sort of “Cheat code” for making Yamato a crewmate– as if all he needed was to evoke Ace and ask to join right off the bat, and then Yamato could just do…whatever for the remainder of the arc. The bulk of his actual paneltime in the story being tied to helping Momonosuke and a fillerish-feeling bomb-removal becomes totally irrelevant a way to run out the clock to the end of the arc because the “work” has already been done. The Ace-Yamato flashback might’ve been hyper-abridged and awkward, but it conveyed the clues it needed, and that’s enough, supposedly.
We could always see Yamato join at a later stage, or even become some kind of Vivi repeat, with the promise to join once things in Wano are fulfilled.
Nevertheless theres still a disconnect for many, with an oft-stated expectation that Yamato would get more exposure in a more fully featured flashback…or as part of Kaidous flashback…Kaidous PROPER flashback…
And Yamato might get such exposure, and might very well join, but its also entirely possible that he’ll join based on nothing more than this, because that’s just Odas current writing style.
You're likely right about a flashback, but maybe this could be as a means to shed Odens identity and become their own person with new found ambitions.