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.

good to know.  I’ve been using an OpticalRendu as an naa for HQP and SQ is pretty damn good.  
I’m just curious about the Aurender architecture 
I recently purchased a Rockna DAC.  I am using it with my Roon Nucleus.  Looking to upgrade the nucleus.  Have been looking at Baetis servers as well as the SGC. The Rockna has the I2S option, but as far as I can tell will only work with the Rockna server.  Are there any servers I2S options that are not proprietary?

@mpomerantz want to upgrade your Roon Nucleus get a Teddy Pardo LPS. Just heard one installed at a dealer and the sonic improvement was startling. Would not have believed it if I hadn't heard it myself. 
@mpomerantz I’m not sure about your DAC but the ultraDigital has dip switches for different i2S configurations, so you might find that yours is viable. It defaults to the PS Audio i2s but supports other options.