Yes, TWFing costs a bonus action; that's payment for the huge increase in your chance to land a sneak attack. No need to withdraw if your target is dead.
Assuming you can kill it in one hit. Otherwise your d8 hp is going to turn you to paste.
Great Weapon Master doesn't let you reroll sneak attack dice (or smite dice); this has been stated.
Debatable, but I'm willing (since they later DID declare it doesn't stack with smite or BA dice) to accept that.
SA longsword will allow a 1d10 weapon (versatile), so 2d6 isn't a big jump.
See below on why d10 isn't a good idea either.
If you're throwing multiclassing in, then rogue longbow would exist. Or the dreaded crossbow expert/sharp shooter.
Hell, ELF Rogues get it with no MC needed. Still only a d8, which is < 2d6.
I don't think Advantage is the most common way to gain sneak attack. Attacking someone who is adjacent to an ally is in my experience. Advantage isn't terribly common on attacks, at least with the way my players play. We don't use the flanking rules either.
You're not trying hard enough.
[sblock=Advantage in Combat]
- Enemy is Blinded
- Enemy is Paralyzed
- You are Invisible
- Enemy is Prone, and you are within 5 feet
- Enemy is Restrained
- Enemy is Stunned
- Enemy is Unconscious
- You have “Help” from an ally
- You are hidden, or the enemy is otherwise unaware of your presence
- Vengeance Paladin’s Vow of Enmity
- The creature is threatened by one or more of your allies (optional rule)
- If the target is affected by Faerie Fire (spell) and you can see them
- You are the target of Foresight (spell)
- The target has been hit by Guiding Bolt (spell), and the casters next turn has not started
- The target is affected by Otto’s Irresistible Dance (spell)
- True Strike (spell) grants advantage on your next attack against the target
- Mounted Combatant (Feat) advantage against unmounted creatures smaller than your mount
- Inspiration[/sblock]
Last, the level 3 ability for a rogue is a damage increase for the assassin. Getting to use different weapons is the thugs damage increase. If they can't versatile a longsword or otherwise jump up from a 1d8 one-handed weapon, then TWFing is still their best damage option, like the Base rogue.
Just like the Thief, Arcane Trickster, Swashbuckler, and Mastermind, to be exact.
The thug should be about using strength weapons, but not any damn weapon he wants. I have no problem with them getting longswords, maces, or warhammers to SA with, but weapons used two-handed (even versatile) should be verboten. Its a step too far and outclasses nearly any other option for rogues.