Mastering MS Office? Well, ... since MS keeps on changing things, upgrading, retiring stuff, changing the look-and-feel of things, ... that is shooting at a moving target with random directionality. Let's just say that some of the answers here reflect doubt that ANYONE will EVER truly master Office. We might be comfortable with most of it. But "comfort" is not mastery.
As to where to go once you are in your comfort zone, that depends on your personal goals. Necessarily, on that subject, YOU are the subject matter expert so you must find your next direction within yourself.