I am open to the stick controlling length rather than buttons, I agree that it should be dictated by timing the push forward, I hope by doing this you have enough scope to bowl anything from a waist high beamer to a short bouncer.
In regard to bowling swing, I had an idea based on the "Golden Tee" golf game, so if you wanted to bowl in swing you could pull RS down/left then up/right and vice versa for out swing (rh bowler). For off/leg cutters down straight then up left/right. Slower balls back then up but not fully up. This sort of control system, would mean you could control the delivery stride with the back motion then release with the forward. Also it would free up the LS to control run up or something else.
With spin I'd keep it pretty similar to how it is now but introduce pace control, maybe have back on RS for slower balls and up for quicker, with left/right for average deliveries, which could also control the flight or drift. So RS down/left would be slow drift, down/right would be slow flight, up/left would be quicker drift and up/right quicker flight. The LS still controls amount of spin, but up it to 5 revs max but make it so if your playing on a dust bowl(assuming pitches make more of a difference than they do now) you could bowl with differing levels of spin rather than having to bowl 3 exact rotations.