Point of diminishing return for a DAC

Get an Audio-gd R8 DAC (ladder DAC) and a ultrarendu streamer/bridge and you will be MANY steps above now and VERY close to a multi thousand dollars DACs. 

The Audio-gd R8 is FPGA based so you will be able to update firmware and improve its sound very often. 

I find A MUST to feed any DAC with a clean USB signal and I dont agree on keeping the Mac Mini. I have the SOtM SMS-200 Ultra Neo and it was a BIG improvement over direct computer USB, even with a W4S USB Reclocker and an Audioquest Jitterbug I had. If budget allows, go with the SOtM SMS-200 Ultra Neo. If not, a cheaper and I heard also good is the Ultrarendu.

My system sounds (for me) better than three multi thousands dollars systems I've heard and, even its not only the DAC, but the whole chain what matters, the Audio-gd R8 was up to the task.