@woots : I appreciate you taking your time to explain your process. It all makes sense.
Here are my thoughts, with the assumption that your Nagra DAC is there to stay, and you keep Roon (I am a 5+ years lifetime Roon subscriber, so I understand). There are two ways to do this:
1) Keep your Nucleus as your remote Roon Server / Core. Get a standalone Roon Ready streamer (no DAC) to “feed” your DAC (I.e Streamer —> USB out —> DAC). Such as the LUMIN U1x. Plenty of good options on standalone Roon Ready streamers/ network transports, but since you like LUMIN sound signature, U1x
2) Get a very good Roon Core / streamer unit (ditch Nucleus), such as Innuos Statement or Zenith MK3 mentioned above. Run USB out from it to your DAC. Yes, Innuos has relatively low powered processor (on purpose, by design), but unless your are doing heavy DSP on Roon, or you have a huge library, it won’t be a problem. With Innuos, in addition to Roon, you can also use their own Sense streaming platform, and you can switch between Roon and Sense with a tap in your tablet or phone in no time