A possible Dark Harvest fix

Simply make lingering/toggled AOE and Dot spells/abilities/items apply the damage at 50% like normal, but only collect the soul if the target is 30% or lower. These kind of abilities are super easy to land that proc because of either the window of time they have or the range to use them. Basic attacks and instant damage abilities should be left as is. An example for clarity, Karthus' Q and Ult are an AOE/multi-target, but are not lingering, so they would proc at 50%, but his E would proc at 30% since it is a toggled AOE.

Morgana for example has a point and click AOE DoT at a long range. It is easy to proc 50% health champions from safety. Garen on the other hand, has to run at them and can only use his ult/E for the smallest amount of extra range to proc. Not that distance to proc should be considered, but at how low the champion is should be.

There are not many chances for the soul to be harvested if your window is 30% or lower. Most laners will either back off and base, or look to heal some other way because 30% is too risky to try and trade. Eventually you may get to where DH procs for at 50% health for 20% health worth of damage and the DoT carries over the 30% remaining health mark. But that point is not until the mid game at earliest since getting the stacks will not be as reliable.