What you describe is pretty much the setup I have. I have a Marantz 6014 that has 10 RCA inputs for 7.1.2. The outputs of my X32 are physically connected to the Marantz inputs, and I can assign those through Nuendo to play back my projects. I also have two more ceiling speakers that I feed directly from my X32, so I can monitor and mix in 7.1.4. The signal is not processed by the amp, and it is not recognized as Atmos - it’s just the pure multi-channel mix from Nuendo.
For playing back Atmos content from the amp, I switch the X32’s 8 XLR inputs and 4 aux outs to the feed from the amp. My computer’s normal media output is fed to the Marantz amp via HDMI and just becomes one of my sources, as does my cable feed, blu-ray, and anything else. This arrangement works out great, as I can listen to mixes in DD or Atmos or whatever format they happen to be in, and the amp will even up-rez the content to take advantage of the ceiling speakers. if desired.
Some lessons learned - mixing -10 and +4 unbalanced and balanced signals is a recipe for hum noise. I’ve had to put direct boxes on every channel coming from and going to the amp to avoid this. For that I used two 8 channel DI rack units. Also I have a large handful of external synths which all have to be sent through DI boxes before going to the mixer to avoid hum. Hum in this scenario is no laughing matter.
Honestly the hardest thing about all of this was figuring out routing. I ended up getting a Behringer S16 unit to give me more inputs and outputs that feed into the X32 digitally. I use them for more external gear inputs and additional outputs for a separate set of 5.1 monitors, and a special stereo fold-down mix for doing live webcasts. (Yes, my studio looks like a speaker garden!)
I feel like I’ve achieved the point where my studio works they way Atmos is designed - as a full-blown sound environment rather than just a bunch of speakers. And there are times that I’ve been doing live-to-multitrack music scoring where the room just absolutely engulfs me as the sound dances around me in all kinds of ways, all under my control. I start to forget about speakers and placement and it all just sounds like it’s coming from every direction and the entire room becomes an instrument. It’s overpowering sometimes, actually!