I’m not certain if I’m understanding what you’re saying, but, if I do, I think you’re turning the truss rod nut in the wrong direction.
You say you can get it flat with no string tension, but it bows with string tension.
You then say that you try to flatten the neck with the truss rod but the nut almost comes off.
If the neck was flat without string tension, adding string tension will cause the neck to bow forward.
To adjust against forward bow, you need to tighten the truss rod, not loosen it as it seems you have been doing.
So try turning the nut in the other direction bit by bit and see what happens...
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