(2) Set: When consuming a corpse, fire a free Corpse Lance at a nearby enemy.
(4) Set: Dealing Poison Damage grants a stack of Plaguebringer, increasing Poison Damage by 0.5% and reducing damage taken by 1% for 5 seconds. Max 50 stacks.
(6) Set: Corpse Lance and Corpse Explosion deal 5000% increased damage and apply The Pestilence. Targets affected by The Pestilence fire a Corpse Lance at a nearby enemy every second and cast a free Corpse Explosion when they die.
So the 4 piece bonus affects only the poison runes which means the build will force you into a poison focus. You also lose the bonus to Bone Spear, which makes the Blight Spear build less effective. I don’t think your suggestion is the way to go. Frankly, Pestilence set is not in need of a rework, but could probably use a tweak on the bonuses since LoD does most things better than almost every Necro set currently.
Blight spear build was never effective at higher torments and especially for gr pushing. The best you could hope for was simply using it for ORoTZ cdr on land of the dead, but even that isn’t good enough for actual gr pushing. The damage bonus was not even close to enough and honestly the coolest part of the pestilence set was always having corpse lances flying everywhere. I always liked pestilancer better, even though its now outclassed by legacy of dreams.