Raspberry 4: best i2s dac?

Not sure I understand your question.

Meaning USB dac compatibility list.

Did you search on the forum?

Yes. N/A. Isnā€™t there any common features, e.g. DAC AK4493, XMOS XU2/3** etc. having certain default values?

Nope, We support Volumio not all DAC brands.
So we (read community) report DACā€™s that work or donā€™t work with Volumio.
Itā€™s your task to determine what your DAC needs to do and find one covering your requirements.

The majority of standard USB DACā€™s will work with Volumio