Bards are pretty much the best at being a thief because unlike any other class;
- They get to almost immediately max out their Sleight Of Hand by picking a proficiency at level 1 and at level 3 picking an Expertise.
- Have access to Disguise, Minor Illusion, and Invisibility.
Disguise while pickpocketing does not drop NPC attitudes,
Minor Illusion is useful if the target is being watched over by other NPCs, and
Invisibility is useful when combat triggers if a pickpocket attempt fails.
A Bard with max Dexterity and items that boost Sleight Of Hand will be able to steal and open anything in the game pretty much from the very get-go. They're pretty much the best main character class due to how versatile they are, both in terms of dialogue and gameplay. And they absolutely excel when paired with other classes like Cleric and Paladin, the roleplay is insane.
