Is it good to max or +4 your EAC? Is it good for a cosmicorb chakram build?
Depends on the weapon. Weapons that can do multiple hits will benefit from it. As for chakrams, it does improve the weapon damage. On a chakram build, it’s your call, but for me you’d better use those slots on different affixes. 
Btw, this is my current set and affixes. I still need Living Force and Adventurer. DMG range still at 653.3K-1. I want to know what I need to add or change to at least reach 3M DMG range.
Any advise will be helpful. 
Pet is level 100 of this 1.
how to get eternal pet?
Kill 199 enslaver, then equip all ur gears with eternalized set affix before killing the 200th enslaver and pray for rng’s blessing.