My left hand is not symmetrical and I didn’t bend the thumb like you mentioned and “extend” other fingers. It was acting more like a tilted German grip.Another thing I used to do wrong was, playing matched grip, I was using 2 different techniques with my hands. On my strong hand, my thumb joint would flex. On my weak hand, my thumb was kept super stiff. So after realizing that, I analyzed and changed my technique in my weak hand to be the same as I was using with my strong hand. Done by simple observation. Working the above exercise, the muscles in my weak hand would burn as it gradually came up to the level as my strong hand.
I started exercising my left hand holding the stick only by my thumb and the area very close to the finger tips of the other four fingers. I practice hitting and pulling up on the stick and don’t emphasize rebound as I want to exercise the fingers. It is like the fingers and thumb are performing a bench press exercise, bending and straightening. I do it to wake up my hand muscles that were not being activated and not as a regular stroke. It helps me.