Where should I go from here with my digital front end?

Can you post all of your system components under your profile?

If you are going to cleanup digital, start with filtered Ethernet cable as your last link prior to entering the streamer.  Also, do whatever you can afford to clean up the last  few feet of power to your components.

I just got a U2 mini and love it. I had a high end digital coax cable, a mogami digital coax cable and a mogami digital AES cable (xlr).  I could not hear any difference between the cables.  I think it is because the Lumin and the Dac are so good.  

I heard a huge difference between my Esoteric dac and my reimyo dac.  I would focus on upgrading your dac.  If you have questions on cables Mike Kay of Audio Archon is an expert in digital music streaming plus he is honest and has stopped me from spending more money on cables that won't make a diff. 

Best JH

I was using a 30' CAT 6a cable from my hub to my streamer and recently switched to isolation using fiber endpoints and a short CAT 8 (LinkUp) cable. The upgrade was significant on my system. Guessing there was more noise than expected on the old line and it was affecting the sound/signal.

Given the distance you are covering with your system, this might be a good fit. It is cheap to try, about $100. There is thread here on AG discussing it and recommended components.

Go here, several folks here have taken my advice and are quite pleased with the results.  Get the fiber in what ever length you require.  EMI and RF will not travel down fiber and I recommend using the fiber on your longest run and terminate with a HQ CAT 8 cable.  Crystal clear!

What is the rest of your equipment? There is a place to show under your UserID. This gives a much better perspective. Cables are very component and system dependent. Typically one wants to stay in a cost range given your overall component sound quality. But the guidelines are really flexible because of system differences.
 

If your other equipment supports higher quality sound upgrade your streamer. If your other equipment supports it you will get a very significant increase in sound quality by upgrading to a, for instance… recommended, Aurender at the $5K, $10K, and $20K component range.