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

Upgrade everything if possible , a good usb or Ethernet cable are at least $750 each , ether regen helps  most critical Ethernet cable is where the hub is 

buy the best dac you can afford ,by far the most important component 

for all detail and resolution, sound signature start here . For your budget I would Highly recommend the Denafrips Pontus2 , ifyou could come up with $4500 the 

the New Denafrips Terminator2 12 th anniversary  but for $2k the pontus 2 beats 

many dacs at 2x the cost.

Here’s the thread I started where I posted more details on the comparison between different ethernet configurations including the optical set up.

@emil I have the same LUMIN streamer, it’s excellent. I also manufacture the ENO filter, so if you’d like to get in touch I can tell you about my experience with the LUMIN.

As others have mentioned, get a switch with its own clock, an LPS for your U2, experiment with the fiber optic route or an ENO filter, better cables, and of course a better DAC eventually. IME every step in the digital chain counts, and ignore the "its just 0 and 1s" trolls.

Does not matter what you do.  You will not be able to improve the sound quality sourcing through Tidal or Qobuz.  The signal is full of jitter from upstream connections.  Once that jitter is introduced, there is no way it can be changed or removed.  Best place to start is with your own local files PCM or DSD.  Put together a collection of your favorite music on a local disk … you can use Tidal or Qobuz for discovery.  Once you have this ‘local’ music collection in place you can revisit the hardware again.  Trying to improve the sound quality of Tidal and Qobuz is a fools errand.