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

Add this to your optical set up.  It made a nice improvement over the wall plugs.  It has two cables and can power both FMC’s.  Optical made a big difference for my system.
 

The Eno filter made a big difference in my system. (Router>.5 ft Cat 8> switch > 30 ft Cat 8 > ENO > Streamer)

@emil 

Another vote for ENO passive filter, they do offer 30 days return policy. 

If you haven't used good feet to isolate your gear, that can be a significant improvement. When I tried it under a DVD player used only as a CD drive it was like I bought a new unit for at least an order more magnitude. And while that was the biggest improvement, isolation under every piece of electronics was audible.

@emil I also use EERO and had Lumin U1 Mini. I highly recommend Purist ethernet cable. It’s a sleeper.
If you can pick it up used its insane value. New retail is around $370. I A/B’d it with Supra ethernet cable(stock and the telegartner terminated versions) and there’s a significant difference. I sold the Supra.
Purist gives you lower noise floor, much better details and soundstage and more vivid presentation. 
If you want to spend more, the Network Acoustics Eno streaming system is very good but it’s more of a different flavor than the Purist in that the Eno does everything that Purist does but in a slightly more relaxed way. I like both the Purist and Eno.

I tried the fibre optic solution with Lumin U1 Mini and while it did lower the noise floor and presented a more detailed soundstage, it didn’t sound as natural as the purist or the eno. I ended up sending that stuff back. I would not recommend a switch as the Purist or the Eno will be enough of an improvement and result in less clutter.