I'm not quite sure what a script assigning ports actually does, but if I need an app to bind a port in some port range, I just start binding ports starting from 30000 until it succeeds.

If it's in a script then that might be difficult, but a racy approximation could be to loop e.g. listening socat with different ports for finding the free port. (Or maybe: parse output of ss.) But if the port use is separate from the port binding, then it might still be busy once the time to rebind it comes.