Yes the Nucleus One is a Roon server (core). It can stream to other devices over the network, e.g., Roon Ready ones, and you can attach a DAC by USB or HDMI to the Nucleus One as well,
Why retire the Mac mini it will serve exactly the same function as the Nucleus One. Both act as core both can act as a streamer connected via USB to a dac, both will serve to other devices over the network. Unless your mini is really old why bother changing?