The optical input will be used for TV.
Concerning the volumio device, I just wanted to know if it was better to use USB or spdif coax (via hifiberry hat board)... in other words, does the RME dac has some sort of galvanic isolation (or whatever the technical word is) on USB and Coax
No, neither, unfortunately.
I just ran into the same problem and decided to use an 8x8 SPDIF matrix switcher (I already had), and connect ADI-2 Pro via optical to my main system.
4x2 optical matrix switchers are available for $30.
Coax caused earth loop problems here, partly because the ADI is simultanously connected to my "headphones"-system via analog;
Partly because the leakage current from ADI's PSU is exceptional high.
It was a bit unexpected that a device that claims to be "...Pro" in it's name took the shortcut to omit galvanic isolation, sparing 2 transformers that do cost $0.50 a piece if bougt in small quantities.
ADI's AES i/o has galvanic isolation, but the common workaround feeding SPDIF coax into AES didn't work(-around), as ADI's AES In is too insensitive.
In the technical standard, galvanic isolation for SPDIF coax is an option, not mandatory, but:
this, IMO, wrong decision causes a lot of headscraping for a lot of people I think.
USB is another story -
the only way to do galvanic isolation here takes a lot of effort by using a separated, individually power supplied circuit with internal optical (or even transformer coupled?) interlink.
Else, with more simplistic approaches, you easily fry the USB chip.
In all configurations that caused earth loop related hum in other devices in my system, ADI's own headphone outs always stayed clean.