??? What are they talking about? The SGCD is a DAC with pre amp, it accepts DSD over PCM (DoP, same quality as DSD up to DSD 128), PCM up to 24 bit 384 kHz, red book CD (16 bit 44.1 kHz).
The Nucleus and Transport i5 run the Roon Server which is all you need as long as those units output the right audio formats through the USB port. A USB DAC does not need to be Roon ready.
In case the transport i5 requires or Small Green Computer advises you need to add another box (potential connection issues, extra cables, potential noise etc.) don’t buy it. The transport i5 has USB interfaces. Either Small Green Computer does a poor job or none at all outputting sound through USB or their personnel doesn’t seem to know what they are saying.
Keep it simple, as few components as possible. Roon provides / developes the hardware with software, I believe it’s the most reliable path to go.
I use my iMac myself, USB straight into the Stellar Gain Cell DAC which simply sounds very good.
I understood the Octave comes in the signature range (BHK, DSD Sr.) so will certainly not be Stellar Gain Cell DAC / Small Green Computer budget.
Even if Octave would be Stellar range, and the new Stellar Phono Preamp price will set the example for any new Stellar series component that Stellar Octave streamer (if it will ever see the market) it will not be Stellar Gain Cell DAC budget either.
Streamers, unlike DAC’s, must be Roon ready certified in order to work with Roon, it remains to be seen if the Octave will be Roon ready registered, at least I have not read a confirmation about that.
Don’t worry too much, the next day you bought an electronic device something new or better pops up, it’s a fact of life. Buy what is available, as simple as possible within a reasonable budget and enjoy the music.