Possibly best persuasion build ever?

If  I play 7 levels purple dragon knight and 3 levels elequance bar wouldn't i have QUADRUPLE proficiency on persuasion checks. Expertise doubles any skill including persuasion, while royale envoy also does that. Their not the same ability so they should be stackable. I mean its kind of like how you can have two channel divinities if you have cleric and paladin.

It doesn't work that way but if a player at my table tried it I might let them get away with it, they need a bone throwing their way for taking 7 levels in one of the worst subclasses in the game

Eloquence bard and Fey ranger, as low as 6th level (3 and 3) or any flavor of rogue plus fey ranger, but that would not come into full power until 12th. You have min rolls of 10 on persuasion and deception (bard route, includes intimidation if rogue route), with both Cha and Wis bonuses.