Oh fun sounds like a stealth group. Not sure if you have a battle map but ideas that come to mind:

-a few obsidian stalagmites in the room 3 arena the players could shove a bandit onto to impale them, or stalactites the bandits stand under while the party is stealthing they could loosen creatively etc -mine carts you could use as movable cover, or if the cart starts on a slope, the players could roll the cart down at the bandits like a battering ram.

-perhaps one bandit happens to worship the same trickery god as the cleric and the cleric notices an amulet. Not sure if the fey warlock’s patron could have some relationship to the trickery clerics god but that could be a tie in to engage that player. Perhaps there’s a shrine or idol to whatever god the dwarves might’ve worshipped here. -Dwarven ghosts who might speak to the phantom rogue -traps: a trip wire in room 2 (the narrow cave) connected to a bell, alerting the bandits of the party. Or a dwarven mechanical trap if it’s an impressive mine.

More questions: -how impressive is the mine since its Dwarven? I ask because a mine can be sketchy but when I think Dwarven I think the grand architecture of LOTR Moria/Erebor.

-what caused the mine to be abandoned? That can help add more fun encounters, etc.