Best Digital Interface

30.3 Digital
  Clocks: Internal, SPDIF In
  Jitter suppression of external clocks: > 50 dB (2.4 kHz)
  Effective clock jitter influence on DA conversion: near zero
 PLL ensures zero dropout, even at more than 100 ns jitter  Additional Digital Bitclock PLL for trouble-free varispeed ADAT operation
  Supported sample rates for external clocks: 44 kHz up to 200 kHz
 Internally supported sample rates: 44.1 kHz up to 768 kHz

Maybe I'm a bit dim, but this section of the manual states that this DAC controls jitter of all formats.

i2s is best. A good DAC will exhibit NO difference
Yes there are many streamers <$1K starting with a Ras Pi 4 <$2C.
No converters from SPDIF to USB.
That would be like putting a spoiler on a horse cart.
The very same for SPDIF to i2s.
I tried until I was schooled.

Usb is a flawed interface. People try to add many tweaks/accessories to try to get it to sound good. Moved from USB to Ethernet and i2s.

Is there a Coax to USB adapter available? Does this make sense?

The beauty of async USB interface is that by desynchronizing clocks it allows DAC to operate at its own independent stable clock.  It does it by signaling back to computer to increase or decrease (upon buffer under or overflow) size of next frame.  With adapter there is nothing to signal back to. Computer cannot provide more or less data since coax from computer to adapter is one way only.  There is no way to tell computer to stream data faster or slower.  Perhaps adapter can have large memory to store extra samples, but it would require big delays.  If you start with coax I would stay with it.