Art of the Con
While many thieves rely on finesse and dexterity you've learned that many times its easier to just fool people instead. Beginning at 3rd level choose either Brawl or Special Techniques. You can use Charisma in place of the normal stat for Attack Bonus and Save DCs for that technique type you cannot change this selection later. Additionally you can choose one Dexterity based skill or tool during a Long Rest this skill or tool now uses Charisma for its calculations until you select a new skill. Finally weapon attacks made with weapons that possess the Light and Finesse properties can use Charisma as the Modifier for their attack and damage rolls.
Rule 1: Never Stop Talking
You know that the moment you stop talking your targets a moment to think. At level 7 choose either Vicious Mockery or Verbal Abuse you immediately gain access to this technique and it does not count against your total techniques known. You may use the chosen technique as a Bonus Action against the current target of your Thieves Mark, but it cannot be enhanced. If you possess the second technique you may choose either when triggering this feature. These techniques can be used as Brawl or Special techniques for you.
Rule 2: Don't Break Character
As soon as your character is broken the jig is up as such you've learned to quickly hop into a character and stay in it no matter what, convincing even those that have seen past your guise. At 10th level you gain proficiency with Disguise tools, if you already have proficiency you gain expertise instead, if you already have expertise then you gain a 1d4 bonus to checks made with your Disguise tools. Additionally you can make a Disguise using an action instead of the normal time required by the Disguise kit a number of times equal to half your proficiency bonus (rounded up) per long rest you. If you are engaged in combat while in a disguise that your enemy would normally see as a ally you can impose a 1d4 penalty to the attack roll as a reaction even if the enemy knows its a disguise.
Rule 3: Stolen Identity
You know that the best disguise is that of someone who already exists and you've mastered getting into character. At 14th level, if you have observed a creature for at least a ten minutes outside of combat you can easily imitate their voice and appearance when using a disguise kit on yourself as long as they are no more than one size category smaller or larger than you. While imitating a creature in this way they are always considered under they max rank of your Thieves Mark even when you are not focusing on them.
Rule 4: There are no Rules
With a mastery of the Con comes one simple understanding, the rules are for beginners, someone of your caliber need not abide them. At 17th level the your Thieves Mark can now stack up to five times and any target of it can be switched using a Bonus Action. Your Pinpoint Strike always triggers against a target of your Thieves Mark.