then re-roll either the d20 to hit or even a damage die?

Just the D20, it doesn't affect damage rolls.

since the attack happens with advantage, it triggers a sneak attack, yes?)

Yep

Is that allowed? Is it broken? Or do I overestimate its usefulness?

Yes it's allowed, no it isn't broken, Yeah you might overestimate it just a tad. It's a good feat but not so good that any build requires it, unlike GWM or Sharpshooter.

Should I instead focus on dual-wielding and get that feat?

Dual Wielding is a it of a flavorful trap. It sounds good in theory but doesn't scale all that well and as a Rogue you will have other uses for your Bonus Action.

I'm picturing this Drow Rogue Assassin

I should mention that Assassin Rogue is probably the least useful subclass. You'll have a decent chance of advantage during your first round but you will almost likely never get to make use of the "autocrit" part of Assassinate since Surprise is fairly difficult to come by. I'd advise literally any other subclass especially since advantage really isn't that hard to gain and you can still sneak attack just by having another melee member in the party.