High-end turntables attempt to remove the tiniest stray vibrations from reaching the cartridge via the base or tonearm. Huge damped shock towers magically transform taps into nearly immeasurable blips and massive plinths deaden and smooth out circular motions.
Audiophiles spend a lot of time on assuring that only the best cartridges are used and then put all of it on top (not the bottom) of a rack or tower where any mechanical vibrations are mechanically amplified by the height of the rack itself. Note in the picture below the great pains to assure that the rack does not move: the tiptoes and spikes minimize what instability such platforms yield but why try to fix xomething that is already broken?
Some mount them midway up walls but the height to which such mounts are attached do similar things to mechanically amplify air-borne and floor-borne vibrations. What's up with that? Why would one spend thousands on trying to eliminate vibrations at the turntable and then only to sit it on top of something that adds the same vibrations you are trying to eliminate?
It's beyond me but it is easy to solve: a sandbox. As mentioned earlier, sandboxes are inherently dead. Drop anything into it and the sand does its job and absorbs the energy. Sand's inherent high mass is not easily set into motion as is a long wall stud or tall tower. So why not use a sandbox underneath your turntable? Great idea...glad you thought of it.
I have built three of these sandbox contraptions and both have worked very well. Each time, I've located a small coat closet, gutted the shelves and coat rod, and mounted a 2x4 box on the wall exactly the width. A plywood bottom glued to the frame and there you have a box hanging from several wall studs in a small space. Filling this box with 50 pounds of play sand to just below the top edge yields a good starting place. Top it off with a 2" thick piece of granite that matches your other table tops and you have an acoustically dead surface on which you can place anything, especially a turntable.
Here is the what mine looks like. I use a piece of 3/4" plywood to sit on the sand and to provide a surface slightly above the sides of the sandbox, a rubber isolating mat on top of the plywood. and then another plyuwood top self that hides the sandbox completely:
As you can see, such a closet is also a great place to store your records adding even more mass to the same studs that hold up the sandbox. The pre-preamp is attached to the wall underneath the turntable and hidden behind the records. I have also added white LED rope lights around the perimeter of the door to illuminate the entire closet. And you can close the door after starting a record to keep stray young fingers from messing with the gear.
If nothing else, I would try adding such a sandbox to the top of your existing rack. It's easy to build but it does weigh in at over 100 pounds before the turntable is added so be careful and make sure that your rack can hold up this much weight.