Having trouble reducing the vibrations on my turntable setup

@arjunm , https://www.minusk.com/products/ct2-ultra-thin-low-height-vibration-isolation-platform.html

How about the Vibraplane which has been proven one of the best isolation devices for many many years. Yes there are newer devices that isolate slightly lower in frequency but at significantly more $$$$ YMMV

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.