Dude we didn't even connect Joyboy to Wano until recently, let's not act like it was a sure thing. Toki going to Wano from the past heavily hinted at that and basically confirmed it in the flashback
The journal? Easy. To inform Momo and Yamato and catch them up to speed to the One Piece lore.
I rather wait 5 more years than to learn stuff prematurely. The Void Century is a laugh tale topic, leave it at that. It will make the reveal a lot more satisfying
We haven't connected Joy Boy to Wano at all. The thing that was connected was that Luffy's power up is connected to Joy Boy, not that Joy Boy is connected to Wano. And I don't know how Toki revealed any of that? We still don't know why she went to Wano. We don't know anything about her.
To inform Momo and Yamato? Momo and Yamato are fictional characters dude, they don't need to learn anything. It is the readers that need to learn and "catch up to the one piece lore". Oda deliberately hiding that information is false hype and a waste of time to the readers.
Also, again 25 years is not premature. Oda is wasting our time with filler. We should have been known about the void century for years now but he refuses to tell us anything.