The case for 5 Acts is too strong.
The Jump Festa message was about Sabo, Vivi, Hancock, intermission content. Even if you think Wano ends this year, there is enough time to go over post-Reverie drama, all achievable in 2020 with Oda's breaks, why would Oda jump into that stuff right away? Wano is such a heavy arc that there is bound to be a breather like Elbaf.
Kaido + Big Mom's flashback with Rocks pausing the action for another month minimum goes where? That looks like Act 4 material. Don't forget all the external actors from Jinbe, the marines, Sword/CP0, Izo and the Whitebeard pirates have to get involved. Even if the alliance loses against Big Mom + Kaido, any of these groups interfering will buy enough time for the Samurai and the Worst Gen to get it together. And I'm not even going to touch all the post-climax content of Wano actually opening, two Yonko falling, more poneglyphs found, etc before moving to the Sabo stuff.
Wano ending in 2020 and Luffy beating Kaido in Round 2 does not look like it can be done in a satisfying manner. The only feasible possibility is that Sabo, Vivi, Hancock all somehow end up in Wano.