Best way to get decent movie sound system with Devialet Phantom 1's

[Snoopy8]

I've got 2 Devialet Gold Phantom's . A Sonos Sound Bar, A Sonos Sub running from an Optical Out from the TV that's split before running into the Devialet's and the Sound Bar.  It works fine, no issues with latency to my taste. 

I want to upgrade the system to be Dolby Atmos, and some rear sound, wondering what my best bet is.

Welcome to the forum...

Not sure why you are using a stereo signal from TV to the Phantoms and, Sonos soundbar and sub. Would have assumed that using the Phantoms with optical would have given the best sound. Or you could have connected the Sonos soundbar and sub via HDMI.

To use rear speakers and Atmos, you will need something to decode the movie sound, e.g. an AVR. Are you prepared to spend more and learn how to set an AVR?

If not, then suggest getting a good soundbar which will mimic 3D sound.