Hey Lester.. ‘hope reply isn’t too late.
This is exactly my setup: pi4 connecting using included USB cable to a Topping 10DS, which is then connected to SVS powered speakers via optical.
I used this tutorial, plugged in as described above, powered up and the only additional step for enable use of the 10DS was connecting to the running PlexAmp web interface and changing the ‘audio out’ to be the D10. It works so well I’m replicating this for another room in the house. ‘hope this helps!