You only need a RoonBridge between the core and the music playing device. The RPi would be used as the RoonBridge. So the setup would look like this (for example):
Nucleus>ethernet>Router>ethernet>RPi (RoonBridge)>SPDIF>Onkyo>Speakers
or
Nucleus>ethernet>Router>ethernet>RPi>USB>Cobalt DAC>Moon>Speakers
I don’t know much about the SooLoos and how you are using it. As I understand your system, the Onkyo has a DAC built in, but does not have USB input. So the RPI would need to put out SPDIF which the Onkyo has, correct?
FYI, I put together a RPi with SPDIF out and this is what I used.
The regular RPi has USB out built in.