I have not been given a final price of the dspNexus 2x8 and there are 3 different versions in work. The 2x8 is first, then possibly a 2x2 and 2x4 version were discussed.
The RPi4 will be running a version of Raspbian Buster OS, not a dedicated OS for music. Roon has been discussed, but not tested since the RPi4 code is in work. The plan is a more generic version of Buster as more than one program can be used. Tested JRiver MC27, REW and a few others for use.
I do not build it, so measurement questions are for Danville Signal. You can contact them via their website.