Sorry to throw some cold water (ice water?)on the parade here but I have exactly the same problem with my throwing. It’s a form of short arming. There’s nothing here that’s been given for advice that will correct this problem, my opinion. And this opinion is based upon my own experimentation was trying to change my arm action.
The problem is simply this is how you learned how to throw. And you’re fighting both motor learning issues as well as physiology i.e. your connective tissues in the arm, shoulder complex have adapted to a certain range of motion, flexibility if you will (internal and external range of motion).
Drills that do not specifically addressed arm action will have virtually no effect on changing the arm action again my not so humble opinion.
I was able to effect changes in the short-term i.e. I’m too old to worry about permanent changes. And these changes allowed me to increase my velocity.
As you and others are probably found out it’s not an easy task.