I would recommend checking if the ARC nut, on its own, is enough to sort out your ER collet problem. I had something similar - disappointing ER collet accuracy, replaced nut with a better one (ARC, not a real posh'un), same socket, same collet, problems went away. I have formed the impression that ER is sensitive to a bad nut.
I have a slight suspicion some categories (not all) on Banggood are the "seconds" that didn't go to named importers... even if you get the same item there are higher odds it's iffy. Other things are absolutely fine. Definitely not consistent.
Hope that ARC haul is all good stuff :-)