Flybacks Diodes are only used for an inductive wires or load when a single pole "switch" is opened and there is no other pole such as in a "Half-Bridge".
In a flyback switch-mode PS, the diode acts as a transient in reverse polarity to the opposite supply rail until it exponentially decays with T=L/R (using diode's approximate R & DCR of the motor)
The Motor DCR [ohms] is what causes the massive current when starting and braking and twice as much when changing direction with the opposite polarity voltage. This may cause premature failure on your SPDT switch with the arcing during switching.
A better method is to use 3 position switched for braking the motor current with a load resistor to 0V, before changing direction. This is partially done by snubbers during commutation and could be used to attenuate arc noise but not as well as a brake position with a power load resistor.
The best method is to limit the current by using PWM to limit the current and thus acceleration for changing speeds and direction.