As your construction is 3D printed, increasing the strength at the point of current breaking will more likely transfer the damage to a new location. The obvious and possibly impractical solution is to solve the jamming problem, rather than to try to power through it.
If it's not practical to prevent the jamming, consider to convert the jamming related breakage to a slipping solution. Engineer a coupling that depends on friction to perform the power transfer from the motor to the auger. Ideally, it would be adjustable, perhaps via compression screws or springs, allowing normal operation with no slipping until a jam is created.
Two disk shapes of metal secured to each shaft, in contact with each other provides the friction for power transfer. A clamping ring that only screws to itself keeps the disks together, but when the output shaft ceases to turn due to jamming, the ring allows the input shaft to continue to rotate.
If this is not monitored, friction will eventually deteriorate either the clamping ring or the metal disks, but it will prevent shaft destruction if properly constructed.