Just looking at your CD player example, with digital coax output - you can’t hook up to the PI2AES because the SPDIF of PI2AES is an output, not an input.
The USB of the miniDSP USBStreamer is bi-directional… have a look at the link I shared and photos and you will see separate TOSlink input and output.
ADI-2 Pro USB is also bi-directional and has both input and output SPDIF connections. But also features auto sample rate switching. Jusis wrote a driver to make this happen.
I have NAA on RPi4 and it connects to miniDSP USBStream by USB cable. The UBS is bi-direction and takes the TOSLink input , outputs via USB to the RPi4 which then outputs to HQPlayer.
At the moment it’s an UpBoard Gateway (as recommended by Jussi)
Tiny Gateway System with UP Board - AAEON
Got a pic here: