Thief rogue in BG3 in general is a little OP with its 2 x bonus attack+hand crossbows. Also a Multiclass isn't gonna destroy anything , like 1 to 6 lvls of lvls of Fighter for action surge and archery fighting style, extra attack and an extra feat / ASI... or 3 to 5 lvls of hunter ranger, for horde breaker, hunter's mark, and extra attack, and archery fighting style. But really if you wanted to 8 lvls fighter+ 4 lvls rogue would give 4 Feats/ ASIs just as some ideas. The most attacks in the game though would be 11 lvls of hunter ranger with the Hunter's volley action / and 1 lvl war cleric. this would give you also divine favor which is better then hunters mark or hex since you don't need to move it and it effects all your attacks, even the ones from part of the volley, also gives horde breaker which works with the hunter's volley action. War cleric also allows you to attack as a bonus action with your weapon if part of an attack action, so before you get to volley you could still be hitting 3 to 4 targets a round. rogues may have the best single target damage, but hunter rangers excel at damaging multiple targets.
Thief rogue is very good, yeah, but I still like assassin better even though it is considered to get much weaker as he levels higher with less useful stuff like crafting disguises...which everyone can do. But that's not my only motivation. I believe the dark urge exclusive gear is designed specifically to give the rogue assassins more bite...A LOT more bite...and I've only seen one of those items. Assassins are designed for big damage on single targets and oh boy do I expect my Dark Urge Assassin to outshine every other rogue class.
Now there are ways to min-max with multiclassing but I hate multiclassing...even in D&D. I loathe the very idea of not reaching highest possible level with one class. Which...I mean probably more relevant in 5e because becoming level 20 of a class can net you some huge life-changing abilities...but I still hate it even when the cap is only 12. I will only do it very briefly via respec once for the achievement then reload previous save from before respec and pretend I never performed the damnable act of multiclassing.
