I remember reading somewhere that if you can afford the loss of space that 2 separate walls with no plates shared is the best....and then I thought what a lot of work/expense/space...

I was designing a video conference space for my employer and needed sound control. I called an engineering firm that was owned by a grad of the university I worked for, and he mentioned extremes of sound cladding all ventilation ducts and runs - both inside and out.

That is serious work