still Yamato has a reason to fight Kaido just like Law had a reason to fight Doffy. and i expect Yamato to be able to hurt Kaido as well since Kaido might not be able to uses his full force haki against his daughter. haki goes soft when the user has some emotional hindrance like Sanji against women.
Yamato could be much stronger than we assume .
Head cannon saying haki has emotional hindrance and using Sanji as example. His haki is the best against women(observation) and he for sure used his best armament against BM to defend Reiju. Sanji simple is not hitting woman no matter if he uses haki or not. Horrible example here.
And we could say Garp had this emotional hindrance but I bet he didn't hold himself a bit against Marco. But he didn't punched Luffy. Is not that he hold back his haki against Luffy he didn't punched him at all.
Yamato can't use Adv Armament like Luffy, otherwise she would get rid of that collars herself. But she is at least as strong(probably stronger) than currently G4 Luffy, because I'm not seeing even KKG one-shotting off-guard Ulti like Yamato did. And this was base form Yamato.
I don't know how strong you think Yamato is but I bet she is exactly what I think it is. Above current G4 Luffy.