Whose training could level up these characters substantially, in a way they're not already getting?

[Warmmedown]

I'm thinking Koby doesn't have Garp to train him. Could someone else train him instead? Garp is pretty much the best person for a non-DF, non-swordsman to get training from, as seen from Helmeppo's power up since early One Piece (he didn't do extra night-time training).
Someone like Shanks could be an option, if it's possible that him being great at future sight means he can develop Koby's. The admirals don't seem like they could train him really. Maybe he could train with Ivankov to get good at kicks.

Helmeppo under Fujitora.