Thanks everyone for all the great suggestions so far. The Nucleus is running solely as the streamer as I have the Roon core on a separate computer in the office serving all of the Roon endpoints in my house. So that one I've done already.
JJSS49 - thanks for clarifying your setup. Good to know you had an improvement going I2S in the Pontus. Did you have the Etherregen in at that time or did you add it later?
Sonic79 - I think your comment that "The last link in the chain before the DAC is the most vital" is very important. That's why I think the DDC would be the best place to start for upgrades. There have been many reviews of that combination online all stating strong improvements in performance. Besides, doesn't the DDC effectively have the same benefit on the signal as the Etherregen?