Toslink and Coax are different mediums but both carry S/PDIF-encoded data. It would seem the question here is not about Toslink being inferior of the 2 but if adding a Toslink to Coax converter may degrade the sound. Good question, I remember some members here reporting certain such converters being responsible for adding gobs of jitter, so they can in theory and DAC-dependent harm the signal . That said, I don't think it would matter much with the Tone board given the ESS chip runs in async mode clocked by its own crystal and has internal jitter rejection means.
As for running from Pixel, you need to try it, you will need to use an OTG cable/adapter. Even if it runs with a direct connection it will likely drain the phone's battery pretty fast, a powered USB hub in between the phone and the board should help with the battery drain and may be the only way to go if the phone rejects the board due to the board power requirements exceeding the phone's limits. You want to use a player app that can output bit-perfect data to the audio device, I think most of not all stock players send audio through the Android audio stack which affects the signal and not in good ways.