Yes I finally saw the secret ending!
And I did it by completing the journal 100% in standard mode! Beat that Luthien!
Sephiroth is really easy once you're on level 99.
Mwahahahaha!
Gummi ship percentage was at a 9% however.

And the secret ending was lousy, by the way. It should have been longer and included a KH3 trial demo and it at least should have featured someone torturing Winnie the Pooh.
I will now search the Final Mix secret ending on youtube.