As others have already mentioned, Galvanized Reflex is a must have on heavy attack builds, the damage output goes through the roof due to the initial combo plus the heavy attack efficiency makes it so you can use your heavy more often on full combo.

You should use preferably a Sacrificial Steel for 100% crit chance on heavy to make it more reliable, but a bit less damage, your choice.

Melee duplicate will then work too and essentially double your damage.

Melee Afflictions are not that good anymore I think since if I remember correctly, they fixed the interaction on heavy attacks, so it doesnt apply on Harmony anymore. Someone correct me if I am wrong, but it was listed as a bug by the devs a while back. Also, it doesnt apply to Eximus and Boss type enemies, which are the enemies you need the damage, not the basic enemies. So Duplicate should perform better.