Welcome!
(BTW, the Lutron devices are technically “bridges” not “hubs,” hence the name. 
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.1. The Lutron “SmartBridge pro” is intended for their professional installer channel. It’s not usually sold at individual retail. That’s why it’s hard to find. I believe they actually sell more of these then the DIY “SmartBridge” models. So it’s not going away anytime soon, it’s just that you’re not going to see it on Amazon unless an installer is getting rid of some excess stock. Or you go to one of the wholesalers who will sell direct to the public.
.2. I suggest you start a new thread under projects to keep from hijacking this one and so you can get individualized responses based on your own set up. That way we can go deep into the various issues about A/V Control. 
https://community.smartthings.com/c/projects-stories
.3. You were absolutely correct: Google assistant integration is a whole different issue than SmartThings integration. At The present time Lutron’s RA2 System is not part of the official SmartThings cloud to cloud integration. It may be able to work with the community “man in the middle” integration, but you should take that discussion to that thread, along with the pico discussions . You’ll get the quickest answers there.