I have a similar issue, I decided to use my right hand for tapping instead. The issue I had with my left hand was galloping (so pressing z then x and only raising both fingers afterwards instead of z - raise - x - raise for a flowy motion). If I wanted to bother to fix that issue, I would bind one key to shift and learn to alternate on that. This might be somewhat caused by a muscle issue - I am ridiculously weak at doing reverse wrist curls on my left arm. My point is that you should figure out why your left hand is weaker, then work on remedying that (or just switch to tapping with right hand). Pushing stamina / speed is fine, but I think it's best to make sure you have a good form first to stop yourself from being bottlenecked (if you aren't already).