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.

The Chord Qutest, Hugo TT2 on up all have galvanic isolation of USB in as well as some other RFI/EMI trix on board... There are technical presentations, given by the designer Rob Watts, of these DACs floating around on the internet somewhere...

Every major computer manufacture has these NUC type boxes, look around... Google is your frenemy...

If you to use a small PC with Roon I've found it's best to use a Roon compatible NUC which will run Roons OS, ROCK which is optimized for Roon and music playback. 

I Like the Pro-Ject S2 Ultra Streamer, and interesting to see the HDMI video out.  Curious how this is used?

i am hoping the usb out will allow a meaningful improve to SQ with all the addl bandwidth available. 
Still, trying to avoid using a separately room located desktop, a pain, to handle Roon. So I may go for a Roon nucleus. Don’t like having to leave desktop on all the time, etc.

On USB an isolator is clearly beneficial. I use an Intona USB Isolator. My DAC initially wouldn‘t be recognisable to the server when inserting the Isolator. Intona suggested to use an usb hub and I never looked back.