Gee, it seems as if you just can’t get turntable isolation low enough. Not easily anyway. Even <10Hz resonance might not be enough for optimal performance.
https://www.ortofon.com/support/support-hifi/resonance-frequency
Apart from totally over the top designs like the Continuum Caliburn and its Castellon stand, it seems as if most designers are happy to settle for just "good enough" isolation.
On the other hand if Michael Fremer’s YouTube videos comparing the Caliburn to the Technics SL1200G are anything to go by that might be enough.
https://youtu.be/qY9YYkqW8ng
https://youtu.be/QKfs5lYZuQk
Or if not, then how much of a factor was the air/sprung suspension of the Castellon stand a factor?
http://www.continuumaudiolabs.com/legacy/caliburn/castellon.php
"For an isolation system to work, the ideal support structure must be infinitely stiff, otherwise the movement of the stand will interfere with the isolation system.
To achieve the best possible result the Castellon uses a nautical turnbuckle system which exerts a significant amount of force to cross-brace the vertical legs and resist rocking motions."