Repetition first. Try to take your mind off the actual stroke and let your body remember the motion. Don't try to increase speed or rotation with force. You're paying quite a bit for the ball. Let it do its job.
You could add triceps curls, dips, shoulder shrugs, crunches, and practice your drawback with your ball. The muscle building will stabilize that form for consistent releases.
I learned a bunch of this from Parker Bohn's book . I recommend it for its descriptions of the mechanics and mentality of bowling.
Bowling: How to Master the Game https://www.amazon.com/dp/0789304945/ref=cm_sw_r_cp_api_IgZXzbAKF5CGA