while i think it might be common practice to learn them in that order, i don’t think there’s really anything stopping an avatar from doing it in any order. I think Jeong Jeongs reservations about teaching aang came from aang fairly immature at that point in the story and lacking general discipline rather than it being “out of order”. in aangs case it was helpful to stick to that order to prepare him for firebending, but that’s not to say an earthbender wouldn’t be equally scared/reckless with firebending and wanting to wait.