Nice system! Depending on your budget, I would look for Innuos Pulse or Aurender N150 or N200. Audition them with a return policy and see if you are able to hear substantial improvement with your Pontus II DAC. A good quality USB Cable + PC is just as important in this equation.
As far i2S, hand shake between two components can be challenging. HoloAudio Red Streamer / DDC comes to mind. I wouldn’t get too hung up on i2S…USB connection with a good cable is just as good with aforementioned streamers.
Good luck!