Not to be dismissive, but there’s no magic in SmartThings. You can write your own Sonos or other drivers. Just as I wrote LANNouncer after getting annoyed at the lack of real outputs from SmartThings.
SmartThings is pretty bad at some stuff. Heck, I’ve had an app in “approval stage” for FIVE MONTHS with no updates. It took them three months to respond to a critical trouble-ticket. They suck really badly, except so does everyone else. But what they do give you, for $99, is multiple radio interfaces, a decent GUI, pretty good basic functionality, a decent user group and better easier expansion than any other system I can think of… and I wrote a popular plug-in for HomeSeer! It’s not that you don’t want to leave SmartThings - I bet the whole community sympathizes there - but where will you GO? From is easy, it’s finding a better “to” that’s hard.