Why do you assume the belt has to be stretchy or even a belt?
I do not assume that. It´s about strech slip that´s inevitably occuring in any belt driven design and with even new belts or "special made" so called non slipery belts too. I´m simply talking about a technical fact.
Each time I changed the connection between the motor and the heavy platter, the sound improved.
Each time the motor had less and less influence on the platter. The platter is massive and lifted by a cushion of air. It maintains a very stable and accurate speed and is very silent because there is no bearing noise. The inertia maintains steady speed.
Please don´t mix up things. Of course it sounds different in your experiment but impression and leveraging about better or worse is 1.) individual/ subjective as long as detected only by ears and 2.) needs to be prooven by measurement if you want to name it "better" or "worse".
Effects are dependend on what motor type is used and how motor speed accurancy is achieved. Secondly it depends on the amount of tourque the motor can provide and the inertia and mass of the platter. All of this results in speed accuracy and wow and flutter measurement. But with building on high inertia only or reducing speed control effects on the platter you´ll generate higher Wow and flutter as well as less speed acuracy. Speaking of better or worse in this perspective and with perspetive clearly on physics it is always the worst one can do.
With low speed accuracy and therefore higher wow and flutter values you´ll recognize a bit more bass on first sight, going into detail will point at there are less details. Furthermore spatiality changes to details are more in the forefront, stage seems to be detailed at first glance but limitied in deepth and width at second view.
With higher speed accuracy and lower wow and flutter values you´ll get more details, more spatiality, stage will become more width, deepth and height, because of more details you´ll say there´s less bass and more heights in the picture painted than before and some people will therefore prefer lower accuracy in speed.