The reasoning I had behind suggesting an herbalism requirement is more or less learning from the mistakes you made and discovering new, harmful, properties. Another route that could be pursued for it is making poison crafting a skill akin to a black powder avenger with their unique crafts. Assassin specifically because it is thematic to the class and with their narrowed skill set (also reflected in way less skill points than rogue levels would get) and shows dedication to a powerful tool for the RP of it rather than having it be hyper accessible.
As for the DCs - a mix of fortitude and will saves could make them more viable. I was thinking somewhere in the 20-25 range for most of them. Or something like 15+ assassins levels. Or something like 10+2x assassin levels to really reward someone putting half their levels into the class. In my own suggested poisons I’d recommend
Paralysis - will save, reflecting on the poison interrupting your decoys system responding to what you want it to do
Bleed - fortitude.
Slow - will. Sort of the same concept as the paralysis
Debilitating - fortitude
Burning - fortitude, half damage on a successful save. You probably could flavor several different damage types too (fire, negative, acid, etc)
A blinding poison or sleep inducing poison also would probably be appropriate.
To prevent people from asking their assassin friend to stock pile them poison, they could expire like gris gris.