Yamato's chances of becoming a crewmate went significantly lower the moment his fruit was revealed to be that of Wano's guardian; in my opinion, Oda exploring this went against Yamato leaving the country.
Also, I may be wrong here, but I have the feeling that Toei may not be able to afford paying Saori Hayami as a regular in a weekly show and wouldn't surprise me they hired her after Oda secretly confirmed them that Yamato wouldn't join.