If you’re using a DSP system (Najda still?), then it makes no sense to use a DAC, you just need a suitable digital out. I2S is a much better standard than SPDIF as it includes a clock. An asynchronous receiver is much more common, and should be the same as I2S if they are well implemented, but with the Najda I don’t think you have that option.
But all of that is irrelevant if you just want a DAC with a Pi - the digital part should be essentially perfect as it becomes the sound card, so the data into the DAC has no timing issues. It can’t be improved upon. Obviously this doesn’t guarantee perfect sound forever - the clock, conversion and output stage of a DAC can all affect the sound. But the pi DACs are pretty good and great value for a small, easy solution.