Only recently this has been discussed on another thread; if you haven’t already done so, you might read through this thread and gather whatever you find useful:
My own take on this is that in the Roon ecosystem you don’t really need anything else as transport than a Raspberry 4 with RoPieeeXL (which includes Librespot, a SpotifyConnect clone). If you wish to connect your DAC by anything different than USB, you’d choose a HAT for the Pi with S/PDIF outputs, or something like Pi2AES which offers in addition to well-implemented S/PDIF outputs AES and I2S.
Especially the Pi2AES is very well-implemented, very well reviewed and may just be all that you’ll ever need. I connect my DAC via USB to a plain Pi4 running RoPieee in a nice little Flirc case and the performance is just fine.