I do quite a lot of residential projects, working on everything from a couple of Connect:amps to huge Control4 systems.

There are rack mount solutions for SONOS products that work well, you can see some of these on this Instagram account: https://instagram.com/ssavprojects

A similar albeit higher end alternative is Bluesound. While it may look like a me too product, the proof really is in the sound quality.

If you do really like Chromecast Audio, take a look at the NAD CI 980 and CI 940. With one of each and six Chromecasts you would be done.