Like I said, if Yamato's performance against Kaido warrants it, I would place her above Zoro and even Luffy.
@Kuro Ashi: the only circumstance in which I would take seriously the claim that Yamato is too strong to be a Strawhat is if she ends Wano
considerably stronger than Luffy.
For example, if Yamato pushes Awakened Kaido to very high difficulty before being defeated. Afterwards, Luffy and Zoro defeat a nerfed Kaido. In that case, I would be more sceptical that Yamato would join the crew because of such a clear power gap between her and Luffy.
But I really,
really, think such a scenario is unlikely.
IMO the
best that Yamato can hope for is pushing Kaido to use his Awakening and then getting beaten at maybe upper-medium difficulty by Awakened Kaido.
I expect Luffy to defeat Kaido in a 1 vs 1 before the arc ends, so End of Wano Luffy will be above Yamato even in what I consider the best case for her.
My median case for Yamato involves pushing Kaido to use his Hybrid form.
I rate Luffy as his performance warrants. It seems he pushed Hybrid Kaido to upper-medium/lower high difficulty. Assuming that Kaido has Awakening (I place Kaido at High Top Tier[+]), Low Top Tier[+] - Mid Top Tier [-] feels appropriate for Luffy's performance against Hybrid Kaido.
If Kaido doesn't have a separate Awakening form, I'll place Luffy at Mid Top Tier.