Continue using the Nucleus as your Core to serve music from the 2TB hard disk. Both Nucleus and DMS600 should have an Ethernet cable connecting them to your ISP router or a network switch if you have one.
It shouldn’t make too much difference sound wise but in terms of ease, keep Nucleus as Roon Core.
If you hook your music up to the Cary you will probably not be able to access them through Roon. For simplicity keep them on the Nucleus.
Plug your Nucleus, the Cary Audio device and a link back to your internet into a dedicated SOHO switch if they are to be in the same room. This doesn’t need to be too fancy to get it to work.
I went with CAT6a. But the above advice is sound. Start with a basic Netgear or TP-Link five port switch. Get everything working to your satisfaction. It’ll be time well spent.