I chose Garling because we don’t know much about him yet, so I wonder what kind of instructor he would be.

Safest bet is Rayleigh d goat

I chose shanks because I think he'd be a good instructor and that Haki would be nice to learn

Rayleigh, because he's not only retired, but a full on veteran combatant. He'll be able to teach me the basic and fundamentals of Haki. Even in later episodes, the basics of Haki are still implemented. Shanks is a close second, though because he moves around a lot and is an active pirate, he would rarely have the time to teach me.

Sengoku is underrated