Just kinda of figured this out, I spent a ton of mats to upgrade items and add aspects and it made literally no difference to my damage.
For example, my ranged weapon damage was 458. I added a bow with +102 damage per second and my damage only went up to 468. So I thought it might be some of the stats on the bows but when I randomly upgraded some armor items I had (I increased +stats and +dex) my damage stayed at 468!
Its like literally impossible to make yourself OP and mow down enemies, your damage can only go so high no matter what upgrades you get. I guess Blizzard is telling us to suck it up and keep grinding…
Edit: My Rogue is lvl 28