+1 to using a dedicated streamer. The Mac Mini can still be used solely as the server.
You are experiencing a major bottleneck by using USB straight off the Mini’s motherboard. The H190 has streaming capability, so I recommend connecting the H190 to your network over ethernet. Then stream from the Mac Mini to H190 over UPnP. As long as the Mac Mini and H190 are on the same network it should work. This will accomplish a few things.
1) Separating server and streamer duties to separate devices
2) Connecting these devices over a network electrically isolates them
3) The streamer can be a low power and low noise device feeding into the DAC