Korrupt
Blackwing Lair Raider
Doing an occultist myself since I want to hit 600 delve, just in case I want to do 40 this league(probably not). Did you pick up the ES on hit Watcher's Eye? Been trying to decide between that one or ES from mana.
ES on hit doesn't actually add much recovery. If you do some napkin math:
9 projectiles/attack * 10 APS * 0.5(average chance for ball to hit humanoid sized target) * 1.5(extra projectiles from good AC positioning) * 25 = ~1700 ES/s. Vaal Pact leech or Vile Bastion are much higher. What would seem really nice is situations where you dash into a pack and take a few big hits, CWDT - Ball Lightning - Slower Proj(or just Firestorm) will get some sustain going before you even start attacking and having to wait for those balls to drop.
ES on hit is no joke dude and it stacks with the things you mentioned ES regen, Vaal Pact, and Vile Bastion. I would strongly recommend Purity of Ice and Physical Damage Taken as Cold x2 with a jewel and maybe helm? That carried me to 1200 last league pretty easily, especially if you have the funds for Atziris shield.