Are the threads on the cup(s) ok? Maybe you could clean up the threads on them using an old steel lockring with a slot cut into it (making yourself a DIY die)?
From what your saying there's nothing you seem to be doing wrong though! If in doubt, get the frame threads chased?
FWIW, when I am being careful not to cross thread (esp with alu frames) I turn the cup in the opposite direction first, you get a distinct point where you can tell the threads have locked together correctly, then I start to turn in the correct direction. Sure you knew this, just sayin'
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