Hi Dan,
I ended up in the exact same situation with my old Mac-mini and bought the nucleus one earlier this week. Happy with my choice (also using USB audio output) though experienced some issues with transferring my Mac Roon “account” to the nucleus one because I could no longer launch my Roon App on the Mac and had no back-up. Eventually found an older rooninstaller.dmg file on my mac, reinstalled the older version and made a back-up. Then restored the backup when configuring the nucleus. I.o.w. Make sure you have a backup and or disconnect Roon server on your Mac before you install the Nucleus. See also Login issue with Nucleus One due to old Mac-Mini's Roon session (ref#BAPLP6)