Interesting theory, and now it makes sense imo
https://www.reddit.com/r/OnePiece/comments/df3bwj
It was never about One Piece's world having 6 moons, but rather having 1 moon and 5 other celestial bodies surrounding it.
If you take only what is known in Geocentric models, there are only 5 celestial bodies that surround the Earth (save for the Sun and Moon) in most depictions. Seriously, look it up, they do not go past Saturn in most of them.
Considering this, Im is most definitely Earth, while the Gorosei are Mercury, Venus, Mars, Jupiter and Saturn. Ultimately, its a question of how of the Sun and Moon play their roles in the series, and clearly Nika is refeferenced as the Sun God, yet is depicted on the backdrop of the Moon
Honestly, really interested to see where Oda ends up going with this.