Volumio is not certified for MQA, so you’ll see the format of the audio container in Volumio (48K), however Volumio passes the full MQA signal to the SMSL if you use USB.
The Audio quality over USB is good with the rPi4 as they have separated the network from the USB bus. (with the rPi3 it was shared)
I would not spend money on a new DAC, but simply remove the Digione. Or if you have a spare rPi built 2 systems :-). Since MQA is already a lossy format, you will not notice a degradation of the sound.
See also: