Assuming your pi doesn’t add untold amounts of jitter, your DAC should still limit what you can get for measured performance.
the allo digione has worked well for me outputting spdif over coax (rca or BNC)
that said, I’m not measuring myself, not having an audio analyzer. Not likely to spring for an AP anytime ever, but might get a quantasylum qa401 or 402 at some point depending on availability.
Certainly no audible (to me) differences between different sources feeding spdif to any dacs I have