Considering how powerful the RPi 4 is, would it not be possible to run a modified ROCK on it, or some other form of Roon core? Sure, some things like up-sampling to DSD512 or very advanced convolution filters would not work, but for more normal use-cases the RPi should be powerful enough.
Combine this with a RPi DAC HAT and a RPi LCD and you get a very neat stand-alone solution. Or a HAT like HiFiBerry Digi+ Pro to get a good SPDIF out to your DAC.
Here is some more info on RPi4: https://www.raspberrypi.org/blog/raspberry-pi-4-on-sale-now-from-35/