Assassination has too many things it needs to set up in order to Multi-dot and AoE competitively.

Engage a pack of ~8 mobs, you want Rupture up on 3 targets for sufficient Energy return. To build Combo Points for Rupture you use Fan of Knives. Before using Fan of Knives you want to benefit from the Envenom buff. In addition you want to apply Crimson Tempest very early so its 12 second dot gets to run its duration.

The conflict starts. Do you spend 5 Combo Points on Envenom for higher Fan of Knives damage, do you spend them on Rupture for higher Energy return, or do you spend them on Crimson Tempest for higher damage? This is too much. Assassination's AoE is ok at its peak when everything is running, but the ramp-up is too long, and the steps are conflicting and not very intuitive. Some steps should be removed.

Crimson Tempest's 12 second dot is generally not going to last its duration before mobs start to die even if applied early. It's a mechanic that isn't very applicable in an AoE environment. However, it's nice to have a finishing move that does AoE damage, so we want Crimson Tempest to be around to spice up the AoE rotation. I suggest removing the 12 second dot and instead let Crimson Tempest apply a weak 24 second dot on your primary target that has the ability to proc Venomous Wounds. This allows Assassination to forego Rupture during AoE. Also, if the Crimson Tempest perk increases poison application chance instead of poison damage, it will be clear and intuitive that it replaces Envenom during AoE.

This way you will still want to switch targets and multi-dot during AoE, but you won't have 3 different finishing moves competing with each other. Assassination's AoE ramp-up becomes Combo Points and getting Energy returned based on getting Crimson Tempest spread to many targets.