Unfortunate that a simple addition to improve direction stability is just not practical in any chassis I can think of: a simple inertia gyro, perhaps built into a vertical wing (tail). Wheel OD can be larger than the tail height if the lower portion is inside the bodywork. Total mass is low, all weight at the rim, strong axle and bearings.
Pre-spin it up to very high RPM with air pressure before starting, might continue to spin for several minutes.
Resists yaw and roll very strongly, completely passive in accel & decel, bump & rebound, and pitch. Maximum yaw control ~ how far back you can place it (ahead of the nose works equally well, but the CP of the gyro should be reaward). Max roll ~ how high above the CG.