I've heard that the requirement for the best ending to have everyone (minus AO) at rank 4 by the end of the 6th day so that they will join you in Diachi's route. Currently, I have every person (again, minues AO) at rank 3, except Hinako at rank 4, at the very beginning of day 6. Will it be possible for me to get the best ending still?
And I'm on my second playthrough. I didn't even know there was a best ending, and I'm honestly only going for it because it gives SO MANY NG+ points. I honestly still like Al Saiduq's ending after beating it and reading about the rest.
Is this about seeing the different cutscenes about the players (e.g. Hinako dancing and Airi playing piano) at before credits when you finished Daichi's route by killing Polaris?
No, you need to turn the world back. But yes, it adds to the ending.
I used the guide on this site for my run to get everyone to 4 before the end of day 6. With a little bit of alterations, you can knock out some AO and Daichi events for the Dera-Deka events. I also went with Osaka during the Megrez and Stake removal battles, since they required more to get to 4 than the guide's suggested Tokyo, who were all really close, whereas Otome and Keita lag behind everyone, including the Nagoya team.