For a relatively new campaign I've built a kensei (monk). At level 7, 8 and 9 (we're level 4 now) I'm planning on taking three levels in swashbuckler (rogue). Now I'm exploring my options for triggering Sneak Attack with this character, in melee.
My character's Kensei Weapons are a longsword and longbow. So with this multiclass idea I won't be able to trigger Sneak Attack with the trusty longsword, or with my Unarmed Strikes, because Sneak Attack states:
Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn’t incapacitated, and you don’t have disadvantage on the attack roll.
When engaged in melee combat, my game plan is to wield a longsword in my main hand and a finesse weapon (dagger) in my off-hand, while still having the rest of my body ready for making Unarmed Strikes (knees, elbows, kicks, headbutts, etc). When the circumstances are right for making a Sneak Attack, I would like to strike with the dagger, without the need for swapping weapons and without taking a bonus action.
My goal is to use my usual attack and damage roll for the dagger when I declare the off-hand weapon attack during my Attack action or Reaction. I would be wielding my longsword too, but not necessarily attack with it on the same turn. Would this work by RAW? Or would I be needing to have the dagger in my main hand for this?
I'm new to the workings of Two Weapon Fighting, so I might be overlooking something here.