I used clay based kitty litter in my speaker stands on recommendation of the stand manufacturer. I think the recommend polymer pellets now.
Clay makes more sense than sand.
The biggest problem you have is that coupling is absolutely pathetic. That's why none of this truly does a lot. Most people add something and then assume it's 1000% magically better, when in reality it's maybe slightly better, if at all, depending on the thickness. Sand for example will help thin walled stuff that tends to move because it's putting pressure onto the wall. With thick walls it does approximately nothing for traveling waves, and only lowers resonances coming up from the weight applied to the bottom.
Here's a thought experiment. Put 20lbs of sand on top of a very flexy piece of of metal. Now hit the metal with a hammer perfectly straight on, perfectly flat. The sand will make the metal contort a little and nothing will move. Now put a 1/2" piece of aluminum panel down and 20lbs of sand sitting on top of it. Hit it with a hammer. The whole thing moves EASILY, as if there's no sand there. The amount of sand (force from gravity pushing it out/down) to stop a solid wall from moving is proportional to the mass of the wall. The proportion necessary to stop the thicker wall from moving FAR exceeds how much you can stuff into the legs of a rack that are beyond flimsy thin. SO the weight alone of the sand is the only acting force, and it isn't a very good one.
If you fill the legs with steel shot, you increase the proportions of weight to stiffness significantly. Suddenly it begins to couple better. The extra weight reduces the resonance. So even while the steel shot can transfer more energy, by doing so it'll reduce the frequency of that energy greatly. There is a loss of energy as it is harder to move more mass. The point being that it's wrong to fear the steel shot because it "could also ring". That is unless the idea of the world's tiniest bell ringing keeps you up at night, but huge audio gear racks ringing does not.
Now let's look back at the example of sand and thin material. If you had an unpoppable balloon filled with 400lbs of sand that you put your turntable on top of, it'd work great. The balloon would be too compliant and therefor transfer basically no energy itself, and the sand would dissipate energy very quickly. How can you do that without a beanbag like style rack? Easy. You simply have to remove the coupling between the shelf and the stand itself. The shelf has to sit only coupling to the sand within the stand. If that is true then all the energy must transfer across the sand, which dissipates energy very quickly. I will however note that sand's low mass does not make it the best for all frequencies. I have my table on top of a box filled with sand. The top of the box is floating on the sand. It works well for small vibrations but does literally nothing for a big one like someone jumping up and down on the floor next to it (my floor is too weak, and I find it easy enough not to jump up and down over crawling the long distance under the house to strengthen it).
Anyways... I hope that helps some. Maybe if we had some pictures of the rack parts we could come up with a solution to improve it? Floating on clay kitty litter? That would have damping.
BTW I think over damping killing the sound in a stand simply means something else needs adjusted. I don't think added information from vibrations getting into the TT is the ultimate way to achieve excitement in your stereo. But I can't argue with an unwillingness to pursue the effort when you're happy with what you got, without too much damping. Some things in stereos become a full-time job we can't all afford to have on top of life..