The biggest problem with the lack of computer is that with an RPi for anything but audio playback, you get user experiences that match what’s out there now. JRiver, Twonky, A+, etc… Roon requires more horsepower in a brain of sorts, and that brain is the Core you must have somewhere on the network.
The right way to solve this is to get you RoonSpeakers with RPi support, you can plug you DAC into that. Also, many hardware providers are looking to embed Roon Core into their devices as well, or you could get a headless NUC to do the same.