So after I get Dark Fencer to 20, what should be my next step? Go for extended mastery or focus on a different class? I'm thinking Warrior for all of those Attack level bonuses but I have no idea if those bonuses are permanent or only there when I'm using the Warrior class?
Your Dark Fencer is going to see a lot of use in hard content and generate EMP on its own, so for everything that's not cutting edge you'd be better off levelling other things, both to unlock some endgame combinations like DF + Arrow Rain III, and working towards Holy Saber/Hawkeye/etc.
Mastering glasses also bestow permanent bonuses, small but they add up.
p.s. if you haven't already, trade 1000 Valor Points -> Gold Moon -> CP, resets every month so you have a day. Solves all CP bottlenecks for unlocking classes and buffing EMP on progression classes.