I think that the all idea behind of this kind of HATs in general is to be at least “look like” an audiophile boards. If someone just want to get some analogue stereo he can do it for 3-8 USD with a numerous of USB to RCA/3.5" “DACs”.
Bad designed audio board will have the same audio quality but for much more money.
Allo Kalli design is what you have to consider going for direct I2S output of VIM GPIO or JLSound I2SoverUSB as a alternative USB based one. But in any case reclock is a must! It’s good to have isolation as well. Best interface scenario for such a board is:
stereo jack out.
RCA SPDIF in (for other audio equipment to use the DAC part of the board)
multichannel SPDIF passthrough (can be directly from the VIM board GPIO for example)
toslink is not needed at all! Toslink is a worst digital interface for audio. To much jitter! This will compromise the board
I have tested VIM with Kali and with I2SoverUSB as well. Kali use FIFO and buffer to make reclock and this is the only valid way when the signal com directly from I2S. I2SoverUSB using asynchronous XMOS based USB with isolation which is the best workaround for recklock in “slave” mode.
For dac chip itself, the newer is better
ESS9028 is not bad, but Pro is better.
The retail price for such a board with isolation, reclock and good dac can be 100-200 (even more with Crystek oscilators for example) depending from the quality of used components.
BTW the picture on your website is Allo Piano 2.1 DAC … not bad at all for the money, especially in combination with Kali