Thanks in advance!
Best answer by melvimbe
Regarding the existing speakers and future outdoor speakers, this support article should help.
Regarding using a playbar or beam, that can work with the connect:amp in one of two different ways. The normal way is the connect:amp is set up as a different room. Then the two rooms can be grouped together to play music in sync, and TV close to in sync. You would not want to play the two rooms in syncs if the speakers are in the same physical room as you'd get an echo effect.
The other way is to setup the connect:amp as rear surround speakers for the playbar/beam.
If you do this, you can't really use the connect:amp to power both your indoor and outdoor speakers.
As far as a subwoofer, you can connect any subwoofer to the connect:amp, except when you've your using the connect:amp rear surround speakers.
Going with a receiver + connect is not a bad choice either. Heos is a different multiroom product, so I wouldn't really try and mix Sonos with Heos products as it could get complicated. Sonos does have a 'works with sonos' program that allows extra features with Onkyo.
In a previous home, I also had wired speakers for rears and for outdoors, and went with a receiver + connect to run it all. The only annoying part is I had to manually switch inputs on the receiver to utilize the connect. It sound like the Onkyo changes fixes that, but I'm not sure if it's live yet. Any receiver will be fine though. Are you thinking of using the receiver for home theatre, or just to power one or both of your wired speaker locations? You mention the Cambridge speakers were in the rear...I assume you intend to use them for surround sound?