Do I need a dac

Roon will not improve your wallet. Some report that it comparatively hurts the sound quality. I didn’t notice that, but it is not for me.  I have never read that it improves ANY SQ.

An external DAC could be worse or better depending on your taste, the resolving power of your system and spending ability.  Most likely diminishing returns  make it too expensive for most.

These guys above are absolutely correct

Only if the DAC you buy is better than your built in DAC. I'm streaming from a Node 2i and was using my Parasound's DAC. I bought a used Aqua La Voce S2. It made a tremendous difference.

An Audio Research Reference DAC would be a significant upgrade. Audio Research DACs are incredible and very complimentary to other Audio Research components. I could not believe it when I paired my ARC Ref 5SE with a ARC DAC! Speaking of synergistic… detailed, natural, dynamic. 

If you are still searching for a "better sound," you should attempt to understand what you need from your system that is missing. You may spend a lot of $$$ on a "better" DAC, but still not get the result you are listening for. It is impossible to make any further comment, you have to drive forward your pursuit of the sound you want.