Hi Alan,
As Grant says I use a PS Audio dac with i2s, so mibbay can help a bit.
Not the Stellar dac I have, but I use tbe i2s input on mine with my Oppo 203 running Roon as a network streamer.
Its a great way to hookup your kit if you can, on paper the BEST way to connect digitally, and PS Audio are one of the few manufacturers who offer it, in fact, they wrote the standard.
From my own use, can vouch for it being perhaps my Dacs best sounding input, so going i2s with the pi is the way to go Id say.
Never heard your Stellar Gain Cell dac, but by all reviews Ive read and its functionality, looks a great piece of kit.
Its PS Audio, so it'll sound great too.
lots of great reviews here.
https://www.psaudio.com/products/ste...l/#tab-reviews
Never used a Pi setup, so can't assist with that, but lots of guys on here have, and they are a helpful bunch.