Having trouble reducing the vibrations on my turntable setup

I think mass is the key. In my case, I have a little flex in my floor and my system resides in a location that gets a lot of foot traffic. I get zero issues with vibration getting into the playback.
Most of this is because I have a Technics 1200G. It alone weighs 47lbs. Bu I also have a solid oak stand loaded with 300 LPs and a 70lb amp.

Use a piece of marble/granite/concrete. Use better feet. It may just be the sum of marginal gains that solve your problems.

I think minus K, vibraplane, and Herzan are kissing cousins, business-wise. If you don’t mind an ugly industrial look, herzan platforms with appropriate load capacity can sometimes be found on eBay. For much fewer $$$.

I agree with whoever said that banging on the plinth or the shelf as a test for isolation  is beside the point, at best.

John at Townshend Audio advised I employ four Seismic Pods in all applications. Being accustomed to using three cones (first the Mod Squad Tip Toe, then the far superior BDR carbon fiber and Golden Sound DH ceramic cones) under everything, I went instead with three.

I discovered that three was generally fine for electronics, and in my case with the Sound Anchor loudspeaker stands bolted onto my 5’ tall planars (two in front, one in the rear, with the rear having twice the weight capacity rating of the two in front. Logical, right?). But for my turntables, four was much more stable than three. With three, the fairly-high mass plinths and platters tended to lean one way or the other; with four, as stable as can be. That might not be the case with low-mass tables.

The Pods aren’t cheap, but imo the only designs providing greater isolation are the much more expensive active platforms intended for use under microscopes, mentioned above by lewm. I've seen the Herzan in action, and if I had the dough I’d buy a dozen! The ingenious design element in the Pod (courtesy of Max Townshend) is the generally-misunderstood damping provided by the rubber sleeve and air release valve that are built around the inner spring. Watch the YouTube videos in which Max explains and demonstrates the design and effectiveness of his creation. Then all will be clear.

The rationale for using only three footers under any single piece of gear is that three points define a plane. Whereas when you add a fourth point you don’t necessarily have a plane. Hence the device can rock back-and-forth on any two of its 4 points. Or, one of 4 is not bearing its share of the mass which shifts a greater than average burden to another bearing. I agree that mounting a 100 pound turntable on three points is a bit scary, but it does give confidence that the turntable is nesting on a plane. After that it’s relatively easy to level the device by adjusting the height of one footer.