I’m not sure that I understand how the breaker is ‘rejecting’ the regenerative current but a larger breaker will not hurt (as long as the rest of the system is capable of handling the rated current). A larger breaker is likely to have a lower resistance which would result in a lower voltage on the high side but I doubt this is the issue…
A quick look at the spec of your motor controller shows that while you can select an exponential response (switch 5), there does not seem to be anything that limits the rate of change which is likely what you need to avoid this issue. As mentioned, the MOT_SLEWRATE parameter is only applied to the throttle input before the steering input is mixed into the output.
The AP_MotorsUGV::output() function that is called in the main loop, applies slew_limit_throttle() prior to calling output_skid_steering(). This then calculates and outputs both wheel throttle settings:
// add in throttle and steering
const float motor_left = throttle_scaled + steering_scaled;
const float motor_right = throttle_scaled - steering_scaled;
// send pwm value to each motor
output_throttle(SRV_Channel::k_throttleLeft, 100.0f * motor_left, dt);
output_throttle(SRV_Channel::k_throttleRight, 100.0f * motor_right, dt);
As you can see, no rate limiting is applied to the steering portion of the output.
I presume that if you go easy on the steering input, that there is no problem? I would get the rover on the ground and see just how much this issue affects your operation. In autonomous/assisted modes there is also the ATC_STR_ACC_MAX parameter that limits the rate of change of steering. Try setting this value to something relatively small and using ACRO mode (requires that you have your heading sorted)