I wonder if there's a raspberry pi project that might fit your needs...
I mean, all you really are looking for is probably a basic power amp or receiver of sorts that hopefully has blue tooth or airplay. Those should easily be found for around $50 or less and even cheaper if you are willing to risk an Aliexpress purchase.
https://www.aliexpress.com/item/Home...5-8645b64756e7
For instance, something like this seems interesting while having a profile barely bigger than a thermostat and seems to be able to stream bluetooth.
The Sonos receiver seems like major overkill if you're running ceiling speakers, but I don't know what's on the market these days. Personally, I've never really found the typical ceiling speaker to be decent quality.