For an auto attacker? You ideally want a default attack replacer as it provides a sizable boost to damage and stacks with WPS, hence why Shaman is a great pick as it has Savagery and Feral Hunger/Upheaval and Devouring Swarm for amping Vitality damage.
On Apostate/Cabalist - your main/only option for an attack replacer is Beronath’s Fury which isn’t bad I’d be hesitant on how strong it would be for this kind of character compared to other replacers.
If you drop the auto attack aspect (so Reaping Strike/Necrotic Edge), Cabalist can make Bone Harvest into a big, cooldown-based nuke however like this old build to show what I mean here, you’d want to look for a more up to date version as it was made before Forgotten Gods.