Paladin wants PTSD - is this balanced?

A Halfling Paladin in the game I just started has a backstory where his previous party was killed and turned to thralls by a lich. The player wanted to incorporate a bit of the ptsd of him not being able to prevent his friends from becoming thralls into the mechanics and I wanted to make sure it seemed balanced.

His idea: Disadvantage against undead (missing would be narrated as he’s having flashbacks to his friends and he’s distracted) but when he hits he gets an extra bit of damage towards the undead creature. I’m thinking add a d6 radiant damage to any hit.

Does this seem like a balanced trade-off? I don’t want to give a boon like extra damage for free but also fear disadvantage is too big of a drawback.

Before talking in detail with him my initial idea would be he’s stunned for the first round of combat with undead or stunned for one round if his party goes down but he mentioned his fear doesn’t come from the undead or his friends being killed, but that his friends were turned and he didn’t have what it takes to finish the job before they did.

Any advice would be greatly appreciated!