A spring suspension engineered into a turntable is the best isolation. You already have that. You probably just need to get a more rigid rack underneath it. Cheap flimsy racks (e.g. VTI) are the absolute worst enemy to quality turntables. At least 4 posts, NO to the stackable / modular shelf designs, look for a wider footprint, don’t go too tall. And filling the legs with sand / shot won’t fix an inadequate rack. Honestly you cold probably do a lot worse than your current support, but it also wasn't designed for high-end turntables. The Symposium racks might be worth a look.
Throwing isolation products on top of an improper rack support isn’t the way I’d go. As mentioned before, you don’t want to stack spring systems. And if your flooring is extremely poor then you’d have to fix that before anything else but sounds like you’re OK there.