Does this method attempting to maximizing Assassin damage work?

I have a plan on how to get a lot of damage out with the assassin from Dnd 5e but I wanted to run the order of actions by you guys to see if it is correct or not.

Assassin 17/warlock 3

  • Apply serpent venom poison to rapier
  • Casts Invisibility via warlock invocation “Shroud of Shadow” (Xanathar’s Guide to Everything)
  • Sneaks to target
  • Casts Hex on the target which is a bonus action and chooses constitution so the enemy gets a disadvantage on constitution saving throws
  • Invisibility drops since Hex was cast
  • Casts Green-Flame Blade for melee attack

If the attack hits then the enemy will have to make a constitution saving throw to prevent death strike and the poison however they will have disadvantage from hex. They will then take:
(2d6 hex + 18d6 sneak attack + 6d8 fire + 6d6 poison + 5 dex modifier + 2d8 rapier)*2

Is the enemy surprised even though I used hex before my attack? Is there anything wrong with this order of events? Is my calculation also correct?