When I make shoot shafts I start with well seasoned shoots like you said you do, and straighten them over a few sessions. I go over the worst bends first then let them rest until completely cool, usually until the next day. After the initial straightening is complete I'll start at one end and heat the shaft while rotating it. I do this until I get good color that I use as decoration but I allso know I'm getting the shaft well heated. I work out the shaft, rotating it as Iwork to the other end. Be careful near the other end because any moisture that might be inside the shaft will be released as steam and WILL burn you. Once I get the shaft hot from end to end I will restraighten it then lay it on a flat surface until completely cooled. Usually by them they will stay straight. If needed a slight heating and restraightening is done. Generally this will set the shafts and they will stay straight.
There are times that a shoot will not stay straight. These I didcard for arrows but they make good stiffeners for Plains style quivers or tomato stakes.