@TimW I have almost that exact setup:
Plex premium (unraid nas docker)
Roon premium (unraid nas docker)
RaspberryPi plus Allo Boss DAC, with Volumio
Roku with Plex on "big TV"
I never use Plex to cast except on the big TV. For the "big stereo," I use Roon and Spotify Connect via the Allo DAC. It works swimmingly.
If you have an Allo Boss DAC, then I assume you also have the Raspberrypi that goes with that? Use that as your network streamer. You can put Moode or Volumio on there. Volumio has an app, Moode has a decent web interface. I haven't been able to cast Plex audio to either. Both suck compared to Roon or Spotify Connect (even this sounds awesome through that Boss DAC!)
I think your DSP might be redundant, and I'd leave that in the drawer.
You can run Rasplex on the Raspberrypi with the Allo Boss dac
https://www.rasplex.com/
But I'm not sure if that will work as a Spotify endpoint...
The Plex server does need some cpu heft for transcoding, so putting that on the NUC is a good idea.
Let me know if you have some specific questions or thoughts. I'd be happy to help anyway I can!