Luffy is the only one that, being the main character, has enough fights and panel time to make everything somewhat logical. He has a random paramecia but basically any new gear makes him quite stronger than before, so his growth is not linear. G4 is completely different from gomu gomu no mi basic usage or even G2.
It's just a free PU like the RS or Zoro's swords disguised as a result of a training or an epiphany.
Enma and Shusui are massively stronger than his other swords, it's nothing to be ashamed of.
It's from the beginning of the story that he keeps switching his swords with better ones.
He does train a lot but he can't be ready in such a short time without an improved defensive capability and more attack power.
Enma even let him hone his CoA.
The RS is the same, except that to use he didn't need a physical training or a new out of nowhere idea to use his DF. He needed (and probably needs even now) to swallow his pride for the sake of his friend, use that technology to help them. A different kind of stress and sacrifice but not an easier one to do.
I'd even say it's more adequate to his character.
He is certainly going to get FS and better CoA, if not advanced one, but that wouldn't have been enough