Ruin Explorer
Legend
And at 14 dex, if you you wait until YOU have advantage to attack you probably wouldn't miss THAT often and your DPR would come all from sneak attack anyway.
You're proposing using both your main Action and Bonus action to spam Help outside of that scenario.
You will thus pretty much never have Advantage yourself, outside of others granting it to you, because Rogues normally use their Bonus Actions to get into situations where they have Advantage.
So your only regularly-seen chance to make Sneak Attacks will likely be when "You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it" - i.e. 5E's quasi-flanking, because you're making zero effort to get Advantage for yourself (and instead granting it to people who gain far less from it!).
Thus you missing a bunch of DEX absolutely will lead to your DPR being lower even when you aren't using your action to spam Help (and doing the latter tanks your effective DPR to an incredible degree - Advantage on a single attack is not a huge deal).
The problem is that a Mastermind isn't a particularly good Face anyway. They're literally not any better at it than anyone else (anyway, they're Hannibal, not Face... just saying) beyond that they can fake an accent without a check, which not exactly something you use every session (it might be if it didn't require 1 min of solid listening to get).
You're much better off just putting Expertise in Persuasion or Deception (or both) if you really want to be a "great Face", than picking Mastermind, and then actually putting your best score in DEX and pumping DEX and using your actions on Sneak Attacks and getting into position for more Sneak Attacks.