I think what you really want is a Sonos Amp with 10 (5) speaker posts, so you could have a center speaker as well 🙂 And of course, a port for a SUB. But that's kind of out of Sonos area at that point, and you're talking about a standard A/V receiver.
But yes, I think you could potentially do what you want with your existing CONNECT:AMP. However, since it doesn't have the necessary 5Ghz antenna in it to receive the surround information, you'd need to still wire it to the Sonos Amp (or have them both wired to your router with ethernet) in order to make that work. But then you could plug in the HDMI-ARC into the Sonos Amp for the front 2 speakers, and have the CONNECT:AMP be powering the rear two speakers.
Bruce