I would not call a thrower build op, it's a decent build. As many of the theoretically fantastic builds it has some problems in the "real world", mainly hit chances and damage resistances. I'm currently in a Tactician Custom run with Honour mode features added by a mod and Combat Extender mod which makes enemies stronger, f.e. + to armor and saving throws, and I feel the difficulties already in Act 1. My main is currently a thrower Barb (i'm wildly experimenting with many builds) and hit chances are ok-ish but not great. I had the same experiences in Act 3 in my Honour mode run with Karlach.
You need to set up for higher chances with the help by other chars or get advantage somehow. I'm also using a randomized loot mod, so I cannot plan with op items so easily, but for you the Risky Ring may be a solution? Possibly it is used on a different char, I gave it at least to my crit build Sorlock. If you like to rely on potions (I hate that), constant slurping of a strength potion can also help (maybe already in use). I'm slowly thinking that a melee Barbarian might be the better way.
BTW if you give Shadowheart the Staff of Cherished Necromancy, she also has good damage, and if you are using Radiating Orb items, she is an even bigger asset to the group, only paired by a Paladin, Wizard 12 or Sorcerer 12. one weaker char in the party then is no problem.
