I'm telling you he reached his prime some time after he grew his moustache... perhaps it was imbued with Joyboy's haki too
God Valley being a huge incident doesn't mean Roger had to reach his prime there, especially when this huge incident was considered the beginning for many pirates starting their own journeys including Whitebeard, Roger's greatest rival, future WSM and one of the strongest characters in One Piece history.
I really don't understand how you could even think Roger was in his prime at God Valley, ~13 years before he made it to Raftel and became Pirate King lmfao. That's absolutely absurd. Let's just wait for the flashback and see.