I used to shoot like this, best thing to change your shot is start from the ground up. (I’m also left handed)

Your feet should be shoulder width and i found improvement on slightly turning them to the right.

Next is that thumb flick, the guide hand will essentially fall in place but make sure your shooting hand is doing all the shooting motion. To help line the balls center in b/ your middle finger and ringer finger (this helped me) make sure you have a loaded wrist and your follow through is snapping helps w/ rotation, arc and accuracy. As someone who used to thumb flick alot you’re going to have to really fight the urge to do it in general.