It's really simple and easy to understand people. A lot of ya'll probably figured it out (or maybe not), but Yamato actually didn't join bc of the power balance that could have been broken in the crew. So what I'm trying to say is if Yamato would have joined, than Yamato would be stronger than Zoro and Sanji which isn't supposed to happen since Zoro is Luffy's right hand and Sanji is Luffy's left hand meaning these two are automatically the second and third strongest on the crew which isn't supposed to change at any point in the story.

The same thing happened with Jimbei and Fishman Island. Jimbei at the time was stronger than Zoro and Sanji all the way up until Wano when all three were pretty relative with Zoro and Sanji finally being stronger than Jimbei. It's honestly not hard to think about when you realize it. Yamato will likely join when both Zoro and Sanji becomes stronger than Yamato. Zoro is near Yamato in power while Sanji is a bit behind Yamato but has disadvantages and advantages against him that could easily tie a battle between them if they did ever fight (which would never happen since Sanji doesn't hit women). A lot of people believe Zoro is above Yamato, but Yamato honestly showed more that says he's a bit above Zoro.

The fact Yamato was capable of holding Kaido much longer than Zoro says something on it's on and the pure fact Yamato has better control of his advanced conquerors also says something. Though Zoro dealt greater damage to Kaido, that doesn't mean he's stronger since haki sword coated attacks tend to be more deadly than haki coated punch or blunt weapon based attacks. There's also the fact Yamato's fruit is likely awakened due to the white smoke seen when he transforms which increases his physical capabilities even more and gives him fast healing.

So as of right now, Yamato > = Zoro > = Sanji