Agree with almost everything. But: a low torque motor (e.g. 2W - yeah, I know watts are joules/sec) simply lacks the torque to affect speed very much. It's barely enough to overcome stylus friction and belt flexion, so it's tiny irregular forces (noise) have little influence on a massive platter.
You note that 'tightness' of the servo is an issue. I do so agree. That's why I use an AC motor - an AC motor sets a constant pace, it doesn't try to correct the pace. The speed of an AC motor can be set very accurately and very stably with a quality power supply.