Okay, so I have the following question. First, I am re-writing the old U1 Sinister Secret of Saltmarsh and there is a situation that comes up and I need to know how to handle it or if it is too overpowered with 5e rules.

So the premise is a 3rd level Rogue (Assassin) is to infiltrate the 1st level party, acting the ally and/or gaining trust throughout the adventure.

At a certain point he wants to attack a party member and then make an escape. How do you handle the sequence of this attack and how does it work or trigger both Sneak Attack (Thief 3rd level so 2d6) and Assassinate (both handling advantage and surprise).

I have come up with the following steps, but this sounds pretty overpowered vs. 1st level party so I might want to change things up. Assume Rogue is attacking with dagger (1d4 + dex bonus)

1. Rogue picks target in party, looking for opportunity
2. Rogue attacks and each character in party rolls surprise (Thief's Stealth vs. Character Passive Perception)
3. Roll Initiative for each character and rogue (you roll initiative for everyone even if they are surprised)
4. Begin actions in initiative sequence.
5. When rogue's turn in combat, if his target is surprised AND hasn't gone yet (lower initiative), the attack is with advantage and critical so damage is 2d4 + 4d6 + dex bonus.
6. If the target is not surprised and hasn't gone yet the attack is with advantage but NOT critical so 1d4 + 2d6 + dex bonus.
7. If the target is not surprised and went before the rogue in initiative order, the attack is NOT with advantage and NOT critical so just typical dagger attack = 1d4 + dex bonus.
8. If the target IS surprised but rolls higher on initiative, the attack is NOT with advantage but critical so 2d4 + dex bonus (no Sneak Attack).

Am I playing this correctly? In addition after the attack, the rogue can two dash abilities (one normal, one bonus) or dash + disengage if he wants.

What is wrong with this thinking, if anything?