Which Clock

I beleive @oddiofyl is correct and only usb will use the DAC clock.  You don’t “need” the ddc but it’s nice to have and will squeeze the last bit of detail out of your source, lower noise floor and make everything sound better. I run a Hermes in front of my Pontus II.  You will want the Gaia if you decide to go that route.

Ok, thanks. Should I use a DDC to convert usb to i2s?

I believe i2s just like spdif uses the sources clock.   You want USB if you want the DAC to use its internal clock