Ideal Connection path to a dac?

Your ethernet connection between the desktop and the node 2i should provide plenty of isolation and the optical cable between the node 2i and Chord DAC even more...  If I were to make a change in your system it would be a upgrade from your node 2i to a better streamer...  Also look at clean power for the router and the node 2i/streamer...  Is your optical plastic or glass, glass is better, and does it have solid connections at both ends...

I personally see no benefit of a Roon Nucleus over any other purpose build PC...  And I would stay away from any direct connect between any PC/Nucleus and a DAC...  If you want a USB connection for your DAC, again, look for a better streamer with a USB connection...

Good luck with your choices...

Pro-Ject S2 Ultra Streamer is USB out. It has a decrap function for your PC. It sounds better than my BluSound. And it does Roon

The one issue I've run into is galvanic isolation on the USB is not always there.  If you go that route, try an inexpensive USB isolator.  Not all DACs will work, and it may not benefit all systems, but

That is, USB can cause ground loops of the analog and the digital kind.  The first causes audible hum, the latter jitter. 
I like the distributed system of the Roon ecosystem, which lets me run a core in a closet, with a Raspberry Pi streamer next to the DAC.

ASRock has some very interesting mini PC's out right now, barebones ~ $300 at New Egg.  Add storage, memory and an OS. This would make an ideal Roon core server, which will run on a free OS like Ubuntu 20.