You could use a networked RPi running RoonBridge and a HAT, that either outputs thru RCA, Coax or Toslink and then connect that way to the Marantz.
Not sure if your Marantz’s DAC will be used tho.
If not then a DAC HAT will allow you to route the signal thru your Marantz.
Hmm, after reading the marketing prose for your Marantz, it seems to imply that one can use the USB input. In that case, a networked Rpi running RoonBridge and outputting thru the RPi’s USB port should work.
It looks like the Marantz doesn’t support USB input from the Nucleus. You probably could use a USB to Toslink adapter to go directly from the Nucleus to the Marantz which would use RAAT. Otherwise, Slim’s method will work too.