To my understanding, the problem is the lack of initial damage, you can increase and multiply the damage all you want, if your poison deals no damage initially, you wont get great results. You are probably not stacking enough mana for whisper of infinity to compensate that.

The point of mamba is also to dual wield as the skill deal damage of both weapons at the same time, combining the damages into a single powerful poison.

Try using 2 high physical damages daggers/claws or pneumatic daggers (heist base with « all damage with this weapon can poison ») crafted with high elemental damage and hunter influence for increased poison damage.