A Copernican View of the Turntable System

Suspended. I don't worry about whether my floor will flex though. I know it will. Earthquakes affect the sound because the building shakes (for those not used to them, earthquakes can be 'noisy'). Having had a setup on the ground floor (concrete slab) before, I know it will move too. That's life in the big city...

Unfortunately for you, seismic design entails making very flexible structures......especially at the joints.....those between column and beam, load-bearing wall and floor.
They are designed to move significantly......virtually like a hinge.
Structure-borne feedback in these conditions is almost a given.
I now understand your problems :^(
Air-borne feedback is very rarely a problem despite what you believe.

I am happy to have the seismic protection for when earthquakes come along. But the resonant frequency of those is quite low :^)

However, I notice the difference between having an isolation platform and not having one. And I notice the difference in loading one differently. And I also notice acoustic feedback from time to time. Wish it weren't so, but I do... But that's just me and if noone else has a problem with sound waves in their listening space, I guess I'm just unlucky... :^(

The result of the structural design for seismic loading creates a building (particularly a high-rise steel or concrete structure) which is continually 'moving' even when there are no quakes.
We are not talking resonant frequency here......we are talking constantly moving structures which are setting up wave patterns of frequencies from the subsonic possibly even into the low twenties.
It's not a good look.....but there are solutions which however would require specialist knowledge.
If I were you....I'd try my Micro SX 8000 or any other heavy unsuspended deck and see if there is an improvement?
I'm betting there is:^)

FWIW... my apartment building is wide and flat and is only 3 floors high. It is certainly not designed with the same features as my office building, which is 15 times taller, and has seismic dampening foundations.

Not sure what to say... My SX-8000 sounds better with an air-bearing isolation stand beneath it than without it, as does my SP-10Mk3 in a SAEC-like metal plinth. Both weigh a short ton. I am currently listening mostly to a very heavy direct drive turntable with stillpoints for feet. The sound is excellent. But I have yet to put an air table beneath it and I am wondering...