Is the DAC in the Node doing the DA conversion, or is my PC/Roon doing the conversion, and the Node acting purely as the analogue out?
Your PC/Roon Core does not do DA conversion. It sends a digital signal out over your home network (which cannot handle analog signals). The Node has its own DAC chip; it can output either digital or analog signals to downstream devices. Presumably, you want your new DAC to do the DA conversion. So, you want the Node to feed the DAC with a digital signal via S/PDIF (toslink/optical) or via USB (if your Node is the latest version and your DAC has a USB-B input). Your new DAC converts that digital signal to analog, then passes the output downstream (to your preamp or integrated amp). You also can connect the Node directly to a preamp or integrated amp (or powered speakers) via an analog RCA cable connection.
The DSP options within Roon Core have nothing directly to do with D to A conversion. They perform functions such as upsampling or parametric equalization. The output is still a digital signal (unless the Roon Core happens to be running on some device that has its own DAC and analog output).