That's a lot of money! Not sure if the following would work for you but thought I'd share another option with you. We use Chromecast Audio's for whole home audio. One is connected to a Denon 3-zone AVR that is used for our three separate outdoor zones. For inside, I have a CC-Audio connected to a receiver and speakers in each of three zones/rooms. All or some of the the CC-Audio's can be grouped together into zones in the Google Home app. We can stream to individual zones or multiple zones. We can also stream different sources (YouTube Music, Spotify, etc.) to different zones. Once setup this has worked seamlessly for our family. The largest cost was the Denon AVR which was about $1200 five years ago. The CCA's (x4) were about $35 each IIRC. I already had speakers and receivers that I had acquired over the years. Now, Google doesn't sell CC-Audio dongles any longer, but I have seen them for sale on a few internet sites for around $50 or so.
I'm sure that the the Sonos system that you were quoted would perform very nicely and maybe in a more streamlined manner for your quoted cost. I just wanted to share what we've done and by my estimate we're only into our setup for about $2.5K. FWIW.