Best Streamer for External USB Dac

You could get a used Innuos Zenmini Mklll with upgraded power supply (important for best sound) for about $1500 or a used Zen Mklll for about $500 more.  Best of luck. 

Lumin U1 Mini...

I’m a SoTM SMS200 Ultra user and have it with the SMS500 power supply also sold by SoTM. It’s a great sounding network streamer and I believe sells for about $1,200 new. I found a used one with the power supply on A’gon and have been super happy. For me, this one replaced their less expensive SMS200 version (not the "ultra" version) so I was already familiar with the great sound.

Most of the more expensive streamers are no-doubt wonderful but you’re paying for the music player software as part of the purchase price.
With simple network streamers like my SoTM or the equally capable Sonore ultraRendu products don’t provide playing software but both work great out of the box with Roon and or Audirvana and half a dozen other software options.

Do you have many downloads? If so why not get a Nucleus (since your using Roon). Connect your DAC to it and let Roon/Nucleus handle the streaming. I’m very happy with my Nucleus and my MHDT Orchid combo with Roon. 

+1 for Roon Nucleus for handling both Roon Core and Network player duties via USB Cable to your DAC of choice.  I have my Nucleus hooked up to Chord Qutest via Curious USB Cable.  Nucleus also has bay that can be fitted with an SSD, so it can handle NAS duties as well for your system.