I think outsourcing the work to a servitor can make sense, but it does not automatically clean anything up or remove you from risk.
If your own energy needs maintenance when stored in a talisman, then negative energy would need maintenance too. Adding a servitor adds another layer, not a free solution. The servitor’s energy, purpose, boundaries, and condition also need to be maintained. Otherwise you are just playing with fire.
How would you know whether the negative energy you’re harvesting has started to affect or corrupt the servitor? And in practical terms, how would you know before things started going sideways?
Another issue is the phrase “harvesting negative energy for later.” What negative energy is useful? What negative energy isn't? Just because something is negative doesn't mean it's useful fuel. Some of it may be usable, some may need transmutation, and some may just be garbage.
Harnessing negative energy is its own skill. Outsourcing it does not remove the need for that skill. It just moves the operation one step further away.