For the first point, it comes down to what Oden wrote in the journal. If Yamato is a walking spoiler for Laughtale, then it would definitely be against Luffy's personality to let them join. However, there are two things that make me think that the journal doesn't hold secrets Luffy doesn't want to know:
1. Roger's whole plan was to get people going out into the world to look for his treasure. Wouldn't he not want anyone to just write all of the secrets down?
2. Everyone who made it to Laughtale stopped their adventure, as they had seen "everything the world has to offer". If Yamato knew all of this, would they really want to go out to sea that badly?
Let me preface the following by saying: I'm a big Yamato fan, would love to have them as a Strawhat, but would be perfectly happy with something else if it fits the story (just want my bias to be clear, lol). Now, regarding the points that you say are working against Yamato:
1. Just because Oda commented on this 11 years ago doesn't mean it'll apply now, as maybe his plan changed over the course of the decade. That said: If it doesn't apply, then it doesn't affect Yamato. If it does apply, does that mean you think someone else with an appropriate DF will join? The options would be Kuma, Kinemon, Shinobu, Kid, or Franky/Zoro/Sanji/Jinbe/Vivi eating the fruits of one of the above. To me, it's more likely that the theory doesn't apply than the second option here.
2. There's reason to believe that Yamato is the guardian of Wano in the sense of fighting Kaido, and that once it's open there won't be a need for a guardian. Also, we don't know that Okuchi-No-Makami is the only guardian of Wano, as at least Orochi's fruit has been stated to be related to the power of the Shogun; Kaido's could also be a guardian (Seiryu is the guardian of the east, after all). If there is more than one guardian, having them all stay could be redundant.
3. I don't really see how this is a point, as it's still the first day after they met. Robin joined the crew the first time several of the Strawhats even saw her. If Oda wants Yamato to join, then we'll get the interactions.
4. Role on the crew is obviously something that Oda can just come up with. The logkeeper is actually a strong point in my mind, as the first thing Luffy said when Yamato pulled out the log was "Ooh, a journal!!". It would be easy enough to say that "Roger and Luffy shared a dream, and this log contains the journey up to Laughtale. Roger couldn't finish his dream, so Yamato will log the journey of the pirates who do accomplish their dreams!"
5. The specific objective is something that, as I mentioned, I would want to be established before they joined. Many of the Strawhats goals only became clear after they joined the crew, though, so I don't know if that's necessary at this point.
Again, I'm biased, but I don't see all of the above as reasons why Oda could choose to have Yamato not join the crew, but all things that could easily be written around by Oda. It all comes down to what Luffy/Oda decide.
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As a follow up: even though Yamato is probably my favorite character since the timeskip, I really feel like I have no read on what Oda's actual plan is. People call Yamato a red herring, and I agree, but I think the "join/not join" discussion is a red herring for Yamato's actual role in this arc, which we still don't know (assuming it's more than just losing to Kaido). If I had to guess a few weeks ago, I would have said not joining for the same reasons you said, and I was mostly playing devil's advocate above. However, the biggest piece of evidence
for Yamato joining is the storyboarding of Episode 982. I made the following video/post on Reddit after seeing the episode:
https://www.reddit.com/r/OnePiece/comments/osoajg
That toast scene was clearly foreshadowing for the ranks of the strawhats changing, and those two scenes made me convinced that it was a reference to Yamato.