You’re forgetting the most important ingredient in mastery of any skill. We see Anakin levitating objects fully ten years after he starts his Jedi training. Luke does this (presumably for the first time) several years after beginning to learn of his ability, and I’m guessing that he could have asked around about the Jedi and their most famous ability during that time. We also don’t know if Ben has been actively helping him during this time like he did during the Death Star battle. I don’t see why he wouldn’t.

The great thing about a time jump in the story is that it allows for the audience to interpret any number of intervening events which help their suspension of disbelief. TFA and TLJ strip this from the audience, forcing them to accept what happens on screen or concoct ever more flimsy justifications.