The whole rig is finally functioning as it should, having been upgraded with a very noisy NEMA 23 for the track function. The rotation works a treat, and all except for the focus are behaving well.
Focus seems to go walkabout rather alarmingly every so often, by which I mean it just chugs off in one direction without having been given any input. I am wary of it damaging a lens, so tend to disconnect it if this happens. Might even swap out the motor for another. May also gear it down even more, possibly the ideal would be a NEMA 11 geared motor for slowness and power.
I have also just purchased a track and the wherewithal to construct a small slider and pan/tilt/focus. This might employ the mechanism from the big rig, but may be better to start again with a new compact version.