| A little neck twist is norma. Both my current basses have a flatter curve on the bass side than on the treble side, as the treble strings are under slightly more tension. You can set up an instrument with this inherent imperfection and get it very playable. The issue sounds like you running out of downward adjustment on the saddles. I had that problem on a guitar and fixed it with a neck shim; there are plenty of instructions on how to do one on here, though I used a thick brass plate on mine because it was BAD.
Also try loosening the saddles to loosen the strings. That might be considered ignominous by some here, as it well affect intonation, but you can make the strings quite a lot tigher/looser using this method without affecting intonation significantly. You could try a little of that, then loosen the truss rod a bit so the strings aren't too high on the upper frets. Looser strings are easier to play, though loose and high versus tight and low is an argument based on personal preference.
But if you really want the strings lower and can't drop the action with the bridge saddles any more, definitely get a neck shim. It'll totally fix that problem, but make sure it's done well, preferably with a flat shim using thicker material rather than an angled shim. Angled shims allow for much more action-lowering using less material, but you lose total contact in the neck joint. If it's neck-thru or set neck, my other suggestion is your only option. | 
