It looks nice and has an integrated mount for my 65” TV. Long term i absolutely want to replace it, but during this pandemic there’s no way I could do anything with this rack by myself.
the good news about that Sanus is that it’s wide with a broad base and not tall. i’d say mass loading the Sanus would stabilize it and reduce resonance. it’s never going to be really stiff, but you can improve it’s stability. what you are trying to do is give your Avid suspension a firm base for it to work.
the cheap way would be a piece of thick (1/2"-1") solid stainless steel on thin rubber gaskets. or a sandbox. both can be somewhat non intrusive to the look of the rack.
better but more expensive would be a Vibraplane air bladder shelf.
but the last thing you want to do is add more in-stability with any sort of decoupling footers. those; combined with the Sanus and the Avid suspension would add resonance and smear to the sound, not reduce it.
as always; you have to try things and listen. an easy way to try the mass loading without spending a dime would be to get something heavy and dense and place it between the Avid and the shelf and listen. if it helps then get something nicer to look at. you could use bricks or a stack of large tiles. anything that won’t move around at all. just to see where it goes. your thought is to see if calming the Sanus improves the sound. it should.
when you are ready to get a better rack, then that new rack should provide the proper shelf for a whole different approach.