I wrote a monte carlo fightning simulator for finding the best sustainable DPR in BG3 EA and it's STR 18 Thief with dual weilding Dragon's Grasp and Handaxe +1. Reasons:
- bugged two weapons fightning
- advantage is very easy achievable every round with jump/backstab
- only one requirement for sneak attack is advantage (no finnesse weapon)
- overpowered Dragon's Grasp dealing effectively 1d6+2d4 fire
- doesn't need Dual Weilder feat and can have 18 STR on 4th level
- 3 attacks per round
- scales greatly with dipping (just use Broodmother amulet and eat an apple for trigger).
Only contender is Wizard with Sapphire Spark amulet till he has spell slots. When the bug with two weapons fightning will be fixed, some STR Ranger builds will be good also.
Ranged build are far away in DPR terms due to being limited to one attack per round.