Graaille

Here is a way to help you maintain the proper thumb positioning throughout the swing and release.

Add or subtract tape appropriately ...

In this so called "set" position in the hole, the base of the thumb is locked in place so it won't
release too soon, in spite of being straight, yet when it does, it does so quickly, but not too early, which is exactly what's needed.

To get an idea of what this should feel like, try this little test: Press the base of your bowling thumb downward against your other wrist, making sure the thumb is straight, thumbnail back ( i.e. thumbnail back as if it were touching the thumbhole).
Now while still pressing, pull your thumb very slightly backward just enough to feel the skin at the base of the thumb "locking" itself against the wrist. This "pulling" back should only be a "tad" 1/8 inch or a 1/4 inch depending on the bowlers
thumb length.

Now try this with your ball. Put the fingers in as usual then the thumb and with the thumbnail touching the back wall of the thumbhole, squeeze against the front rim with the base of the thumb just as you did before. The still squeezing, pull that thumb back just enough to feel that skin at its base lock or "set " itself against the rim of the hole.

Next let the ball hang by your side, still keeping that thumb in the "set" position. Notice that now, even with the thumb straight, the ball does not slip off. That skin at the base of the thumb's base stays stationary, keeping it in place.

Remember, the thumbhole should be at or near it's proper size for the occasion (i.e. add tape or subtract it if necessary)
You can try this at home - a cushioned area to roll a ball into from 4 -5 feet away from a kneel down position -
e.g. kneel down on the non bowling side knee on a pillow or cushion for comfort and extra clearance from the floor, with your slide foot flat on the floor, just like you have slid than stopped and knelt down in that position, now grip the ball ,set the thumb, do 2 or 3 backward swings for a pendulum action and let the ball release ...

This " Thumb set" gambit might be what you need.

Caveat, depending on how tough your skin is there, I would recommend that you temper the time intervals between shots so as to avoid blistering - i.e . like you were in a match against at least one other bowler.

Also, try to lead the downswing with your ring finger to avoid early turn ...