Hi Jan - Congratulations on your achievements. That's really cool. I'm not very well versed in the whole Car Audio world... I'm only familiar with what my pal, Wes Phillips (before he was my pal), covered in some of his Stereophile columns a number of years ago, and a couple of pro installations that a friend had put into his Porsche.

From what I've read, I seem to recall USB DACs having very poor jitter performance. This might be enough to compromise your sound. Over the years, several components in my various main rigs employed (and still do) jitter reduction. I know high jitter is not good for good sound. Hopefully, someone more knowledgeable will chime in and assist you, but, I'd be careful about the choice of USB as an interface.