this can be done with RTI and Sonos Connects. We have done houses with 19 zones, with the same setup.
(4) AD8x and 8 Sonos Connects. To save on there "budget" you can drop down to 3 AD8 and less sonos connects and always add more later. If they need more connects then its a different ball game with the "budget". CLient needs to understand that.

For the outdoor zones just add a crown or some other powerful amp to get more power.

The most important question is how are you with RTI programming? If you are new or don't have experience programming these big systems you can have issues and client will think the system sucks if its not working right.

Regarding what streamer works best with RTI. They are a ton out there but some missing other things then other, like searching inside RTI, etc
We stick with sonos and make the client select there Favs from ANY service and it shows on our RTI interface. Very Simple and clients love it.
Yea they (sonos) broke integration with RTI but RTI released a fix under 24hours. That was only time integration was broke where it effected RTI.

If you need help programming or want to sub out the programming DM me.