In the process of tracking down the source of those Reversal zits, I noticed the motor mount flexed slightly as it reversed. That could produce a bit of backlash, so I added a quick-and-dirty support strut under the motor.
It’s a threaded standoff with a screw in one end. The nut (secured with a dab of Loctite) lets a wrench do the height adjustment. It just stands there, held in place by compression loading.
Unfortunately, it really didn’t have much of an effect on the problem, about which I’ll say more in a bit.
Stepper extruder design has advanced during the last half year, so I may print up the latest iteration of Greg’s Extruder. There’s also a beefy NEMA 17 on its way around the curve of the planet that might suffice as a direct-drive extruder motor along the lines of the MBI MK6 StepStruder motor, but with lower winding resistance for better performance.