Was my English too bad?
They use two small DC motors to slowly move the mirror platform. It seems to me like this would be more suitable for accurately positioning a laser beam (although even that device may not have enough accuracy?)
I didn't understand how the servo motor was connected to the stepping motor in your example,
I will google for the keywords you gave me.
To get a finer, smoother adjustment than a direct mount to a stepper, you could mount a cam to the stepper (or a servo), and use that to tilt a pivoting laser mount. Or use the stepper to drive a screw (or even a micrometer, for really fine control).
hobby servo can make ~426 discrete movements with in its ~190 deg rotation range
The only question is why?
Unless you have an uneven track, not clear why you need a vertical-axis mirror, unless that is part of the experiment?
The closest thing to a computer-controlled galvanometer is a hobby servo motor.