There was an article at the American Thinker recently:
https://www.americanthinker.com/articles/2020/02/q_the_silent_war_continues.html
Re. "The best is yet to come", it states:
"The President debuted this message on January 28 by concluding his massive rally in Wildwood, New Jersey with these exact words. However, Q followers were already familiar with the slogan, because Q had dropped it at 2:26 that same afternoon."
I find first occurrence in Q's post 2867 about a year ago (2019-02-22). Don't know when POTUS indeed used it for the first time.