It was acceptable without a payload, at 6min/mile & 9 min/mile.  It burned 4359mAh for a very high 416mAh/mile.  After pushrod adjustment & algorithm improvement, it's still far less range than it used to be, so perhaps the slight toe out that resulted from not having enough pushrod to center the wheels killed it.  The right wheel pushrod is too short.

Oscillations happened randomly, seemingly from rough terrain.  The thought occurred of using a quaternion for determining heading, to compensate for rolling.  It didn't jump out as a killer solution because it wasn't a killer solution in quad copter days.

Another tweeking of FIR bandwidth & rate feedback without FIR didn't work.  The bandwidth of 0.9 came from bode plots & the vehicle's natural frequency.

The next step now is replacing the servo saver with the methuselah of servos.

https://hobbyking.com/en_us/turnigytm-tgy-615bl-brushless-ds-mg-servo-12kg-0-08sec-60g.html

The mighty $43 Turnigy™ TGY-615BL standard size, metal gears, brushless.  Steering burned up brushed motors too fast to justify the cost of any high end servo.    Metal gears were unavailable in any standard size servo.  Early brushless servos appeared years ago for over $100, only in giant size & only with plastic gears.  Turnigy now has a complete line of brushless servos at dramatically lower prices.  It might obliterate all the investment in algorithms for any reason other than imitating Lars.