Best Digital Interface

If your transport is providing S/PDIF, it's never a good idea to convert this to USB.  The target DAC will have to re-convert back to S/PDIF and I2S anyways, but now you have timing problems since USB is really a data packet interface and not an audio/timing interface.

The RME --might-- be re-clocking the S/PDIF data, but the S/PDIF data from the Node 2 is already clocked using the sample rate of the original music data.

I'm not familiar with your RME, I never heard of a dac that didn't reclock S/PDIF. I'll take  look at the manual.

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.