You can try adding a serial of diodes in oposition to have reverse. 2 diodes low the voltage by 1,4 volt...etc. You will notice a lower full speed but who cares about. You can add several diodes and will have a lower voltage for the same angle of the stick. This doesn't add steps of course just works as a fixed voltage reductor that will not change with the current. Diodes must by 100V 5 A minimun.