A level 4 Thief with the level 4 two Weapon fighting feat will do 3(1d8+Ability score + Proficiency bonus) Per round, not including sneak attacks
I thought that the issue was only that attribute damage is applied to offhand attacks damage, even without the TWF fighting style, as opposed to the attack roll. Correct me if I'm wrong, but you don't add your proficiency to damage rolls usually.
Thieves are still dealing a lot of DPS either way though, so you aren't wrong about that at all.
My fear is that the changes in BG3 are going to make the Thief outshine the other subclasses. I know that things are WIP right now, but currently it feels like the thief was compensated for the weakening of cunning action by giving it the extra bonus action, but Arcane Trickster was left in the dust. I usually like to make build choices based on what fits my character's style, as opposed to just what's optimal, but it's harder to feel good about choosing a less optimal build when there is that much of a gap in choice balance.
