Server/Source Recommendation For Denafrips Terminator

Just picked up a Terminator and quite like it so far. I've an old Cambridge 851C that I'm using as a transport for the couple hundred or so CDs that I haven't ripped, and an Aurender N100H along with NAS totaling around 7TB. Files are mostly FLAC, WAV and about 10% are DSD.

I'm looking to improve on the sound quality of the N100H. I don't have a real issue with the Conductor App functionality and no real desire/need to step into ROON.

However, I am looking to add some input flexibility and not be tied solely to USB. The Denafrips has the usual suspects when it comes to inputs and it also has I2S, which I used way back when with some old Audio Alchemy gear (yes, I'm that old).

I've zero interest in streaming and zero interest in buying a unit with an inbuilt DAC.

Is it worth trying to find something with I2S connectivity, or are the "gains" too small to worry about?

Can you recommend a music server that wouldn't constrain the potential of the Denafrips Terminator and could be found on the used market for around $3000 - $4000, perhaps a little more if needed?

Looking for feedback from people who value (or not) the I2s connectivity and who have some experience with this DAC and a variety of sources. 

Many thanks.

Rooze