If you want to get the top resolution (i.e. DSD) then you will use USB. If you do not have any DSD files then coax or optical is fine. BTW- I normally have one USB driver installed. I can't imagine why you would need more than that unless you are going to use multiple DACs.
The uDSD is a DAC. Why would you send it through another DAC. Why not hook the analog outputs straight to your amplifier? So to avoid using USB, you send it through anther USB and then convert it to spdif and then an additional DAC converts it again <tilts head like a little dog>.... kind of confusing to put all those extra conversions... all you need is the one USB driver to whichever is your end of the line DAC.
I have a multitude of dacs which rotate through my setup. They all have certain characteristics, or I just like having more gear to play with. Rather than install usb drivers for every single one, it's simpler to just use a single usb-spdif bridge to feed the dacs. Some dacs also have lousy usb implementations, or perhaps none at all.
There really isn't an "additional" conversion step. Even when you use a dac's usb input directly, it is still doing a conversion internally from the usb protocols. Essentially, all usb inputs are converters of some sort before feeding into the primary dac inputs.
Additionally for me, the NuPrime driver only works with one device at a time, but I have both the uDSD and IDA-8. Since I don't want to reinstall drivers each time, and I do take the uDSD for transportable use sometimes, it's easier to leave that as the one installed and then use the coax output to feed into the IDA-8 (and overall that's actually fewer conversions steps).