Best Digital Interface

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.

I believe what your original inside outside statement means is that the RME will accept an external clock signal over the SPDIF interface (usually a BNC connector but does not have to be). If using the USB connection, then the RME defaults to its internal clock, which it does with any of its inputs "unless" there is a clock signal present at an SPDIF interface, which is probably set by a menu choice or internal DIP switch?

I believe what your original inside outside statement means is that the RME will accept an external clock signal over the SPDIF interface

The S/PDIF interface requires the data to be clocked from the source.  This is the only way it will work and it is part of the interface specification (the digital pulses representing data will need to be transmitted using the timing of the original music data, such as 44.1Khz or 96Khz, etc.)  However, there are some DACs that will re-clock those digital pulses in effort to reduce jitter and increase accuracy.  Sometimes this provides an improvement, but not always.

You may be getting confused with DAC/transport systems that use a "master clock" for S/PDIF.  In this scenario, there is a separate device that generated exactly timed "clock pulses" to BOTH the transport and dac.  The transport and DAC do their audio encoding/decoding based on the timing of the pulses from the master clock device.  This is usually a closed proprietary system which requires the transport, dac and master-clock devices to all work together.  This is an option usually on $$$$ level equipment.