I do use a polyester ball today. But, over a decade ago when I returned to the game, I simply used my strike ball for spares and threw it semi-back up ball. I only switched to a polyester cover because changing thumb slugs, or fixing cracked thumb holes became bothersome - polyester balls are cheaper. Anyway, it became a habit when I picked up a spare ball to release it flat.
I just need to get over this mental hurdle and try to throw the same way as my strike ball with spare shots - need lots of practice and adjustments. I use a modified "4th arrow" system for many years and never even tried to make an effort to learn to hook at spares with strike ball.
What kind of wrist support are you using? Is it adjustable?
Power Tac Plus by C300. I do use the old school Roby's original without the metal on the palm side. I lose the feel for the ball on my hand, but it helps keeping me playable for 3 sets.