@Tomboli I would try to connect the Nucleus directly to the Marantz via HDMI and see if Roon recognises the DAC in the device and can use it accordingly.
If the direct connection works and the sound is good, I would do it that way. But see:
If it doesn’t work that way, then a streamer, e.g. a Bluesound Node, might be necessary in between. And from there via RCA out analogue into the Marantz. But that would be a compromise in my opinion.