If you connect an external DAC directly to the computer running Roon Server, you may get subtly different sound quality. However, since you have expressed an interest in sound quality, I suggest following Roon Labs’ recommendations:
They wrote the software, so they should know how to configure it for good sound quality. 
There’s a very active “bits-are-bits” crowd in these pages who will disagree, and that’s fine. Each person has their own experiences.
I also posted a while back about a MOCK (My Own Core Kit) solution that’s super easy to set up, tiny, and works beautifully. My informal Roon benchmarks show that it’s about 80% as fast as my 11th gen Intel Core i7 NUC:
Unlike the Nucleus, it has an internal fan, but the computer is so small, you can put it anywhere. It just needs power and Ethernet. Shove it behind your ISP’s router and forget about it.
With Roon Server moved out of your listening room, the sound quality discussion moves back to your choice of endpoint. There are a huge number of certified 3rd party options here:
Lots of fun DIY projects too, including RoPieee, DietPi, GentooPlayer, and AudioLinux.