If you have the wrist strength to bowl without a thumb in the ball you have the ability to bowl with the thumb in. I grew up all my life bowling without my thumb, was a 200 average in my teens before I stopped bowling. I came back to the sport about 1 year 1/2 ago and started bowling again without my thumb in the ball. about 6 months into it the pain in my wrist became intolerable. I took a month off and then finally decided I was going to bowl with my thumb in the ball to alleviate all the stress I was putting on my wrist.
I am almost a year into bowling with my thumb in the ball and I still don't have a proper/consistent release, it was VERY hard at first to learn to get any hook on the ball, it took me about month before I got any type of consistent results.
My average has been climbing steadily ever since I started putting my thumb in. I went from over 200 to barely breaking 100. That was REAL tough for me to deal
with and extremely frustrating at the beginning. There were a couple of times I said I couldn't do it and almost gave up bowling.
#1 thing, is not to expect the kind of hook you are getting without your thumb when you are bowling with your thumb. Don't compare your old hook with anything new you are trying to do. Once you get that expectation out of your head you will do better.
My average is back up to around 170 right now and I am just starting to get a little more versatile in my lane play. I expect by the end of the season I will be up around 180 or 190. Almost back at where I started a year ago! It was totally worth the effort, I can play with very little pain in my wrist now, I can play on almost any condition that comes my way and I am WAY more accurate on my spare game. While my average was higher before, I am a much better bowler now, even though my average isn't near as high.