Its true, full es is pretty bad in pvm. Even with 3.6k mana, it instantly gets deleted by manaburn mobs, it really doesn't matter if you get it any higher to 4k+ either, at the most it buys you time to full reju then save and exit.
The problem lies with the manaburn bug itself, quote:
Due to a bug, melee attacks burn 256 times more mana than intended, resulting in emptying mana virtually every hit.
This is due to a combination of two bugs. The first is because the engine subdivides each point of damage into 256 bits of damage for better rounding accuracy. The Mana Burn code applies this multiplication twice, so all Mana Burn Unique monsters deal 256 times as much damage as they should.
The second bug that increases this damage to 512 times results from Hell difficulty applying a -100% penalty to all resistances. This includes the resistance to Mana damage, despite there being nothing that could raise that resistance in game. Thus, all attacks that drain mana drain twice as much in Hell.
Source:
https://diablo.fandom.com/wiki/Mana_BurnThere is just no way to go around it atm with full es build for pvm.
I know previously people use like a programme to alert for manaburn mobs but i would caution against using it especially with tightened security by blizz.
Very informative, I gave up on ES in pvm yes.