Dragging this back to the topic in hand, I’ve found that practicing in front of a mirror has done wonders for my left hand technique. It may sound weird but you can see the position of your elbow, your fingers and even the thumb too. (Think this was a Geoff tip, can’t remember)
I’ve also found being conscious to get the right thumb position has improved my fingering too.
I’m actually a fan of the Simandl book as I like the way the exercises build on one another, but I can see why they may be frustrating (and in some places not musical).