The only reason for Yamato joining is to sell some mousepads. Her joining the crew would be an insult to every other member, who Oda actually gave a shit about when it comes to having some relevance to the plot - Yamato hasn't got any in comparison. She hasn't incited anything in the plot and has no real hand in resolving issues in the plot. Her contributions the story overall has been sub-par and as far as giving us the readers anything (emotional, lore-wise, or even entertainment-wise) she has seriously underdelivered.
Her backstory was... essentially nothing at all, and on top of that it didn't give anything to the story or even Wano in terms of adding depth (compare that to Jimbe, Robin, Franky, Sanji). In fact as of the SBS where Oda basically retconned Ushimaru's existence as far as the narrative is concerned, I'm starting to believe that Oda wishes he never drew chapter 1024 in the first place.
If Yamato were to join, she would be the most unimportant member, as she has no unique skills or character traits to bring to the table. Seriously, go through all of the strawhats' personalities and tell me one area she wouldn't just be copying another member at any given moment:
-With looks, being an attractive woman - we already have Robin and Nami for this, might truly push Sanji over the edge of insufferability.
-We see a new island - She copies Luffy's (previously unique) excitement and wonder at the new adventure.
-'Stupidity' gags - She copies Luffy, Chopper, Usopp (Seeing Franky's upgrades, or believing Usopp's blatant lies, etc).
-Strength-wise - We already have the M3+Jimbe for reliable strength, any more and we're taking too much from the weaker members, if Oda continues prioritising strong members in terms of screentime.
Look at other SHs introduction/establishment arcs, they were built around them completely (barring Jimbe who had three arcs that together make up for it imo), looking at Yamato she doesn't fit the criteria. We need variety in the crew, not the same members with the amazing addition of sideboob.