The last piece finally dropped into place.
I had largely, but not entirely, fixed the wobble in a specific pattern I’ve been testing against (a snowflake with lots of sharp corners and turns). Adjusting belt tension, modifying acceleration and other settings, updating the drivers and power supply all contributed to making it better, but there was still a small wobble depending on size of the object and speed.
The final mirror (on the laser head) used really bad quality setscrews, adjustment hardware and supplied hex/allen wrench. After re-re-re-adjusting one too many times, I stripped the head of one of the setscrews.
I went to the local hardware store (“Ace is the place,” :)) and bought new setscrews and adjusting screws (and wrench). Tapped and cleaned the adjustment threads and put everything back together. Took about 1/3 the time to do the alignment.
No more wobble, regardless of speed, size, and acceleration. I couldn’t feel any movement on the final mirror, but it was apparently there.