didn’t you mix some things? The board you have has a SPDIF interface and a DAC. If you want to use USB, you have two options. You can use a cheap USB to SPDIF dongle (but in most cases they have optical output and no coax). If you want to use the I2S pins, you need a USB 2 I2S receiver. These interfaces are build with XMOS chips in most cases. But in this case you have to bypass your SPDIF receiver, so you should have the schematics of your board. I buy my XMOS interfaces from DIYINHK. But you have to pay about 60 US$ for one board!
I don’t think I’m mixing anything. I have a DAC board which only has a coaxial input going into the DAC receiver. From there goes into the DAC chip.
Unfortunately the designer didn’t leave the other receiver inputs available to be used, as you could add an optical input.
Cheap USB interfaces give you a poor audio. For I2S you need the DAC to have I2S pins, and my DAC has them. To add an usb source I will need a switch, which I’m planning to do with reed relays. Of course I have the DAC schematic.
And yes: a good interface costs a lot of money, more than my DAC.