Do upgraded LAN cables/purifiers/switch box make a difference. What is recommended, if so?

I too would think upgrading the dac and streamer is money better spent. I have a LUMIN U2 mini and looked into the separate power supply. All reviews said there was significantly less noise as a result. So, make sure your future dac and streamer have isolated or separate power supplies perhaps. Seems like that’s where a lot of noise comes from. Good luck and very nice system. Very revealing so might benefit from improved power isolation. 

@devinplombier 

Your ability to contradict yourself in a short paragraph is prodiguous. So if bit perfect ethernet doesn‘t eliminate EMI/RFI how can a conversion to optical defeat it?
A little hint: because EMI/RFI travels along the bit perfect data on the ethernet and has no means of transport in optical DOH! Hence different cables suffer from different susceptibility for EMI/RFI incursion and filters on ethernet have a task to accomplish!

And btw: depending on the quality of converters chosen, optical conversion has its own problems…

Think in future about making apodictic statements that are patently false!

Noise and timing are the two main issues affecting streaming quality. Take care of your noise issues, lower noise floor means higher resolving/transparency. Take care of your timing issues, far closer to a continuous analog wave form, this results in more analog like sound. As for the equipment, cabling etc, everything matters, any weak link in chain affects the entire chain. 

Now as for where to start. So lets say you go for dac first, problem is if you have crap going in dac is not performing to highest potential. You can also start at the beginning of chain, so feeding dac with lowest noise floor, most precise timing, if dac weaker link not getting full potential of feed to it. 

Point is I've found every single link in chain important, just a matter of priorities. I'd suggest starting with dac and streamer as the most efficient way to achieve high  quality streams. Spend absolutely the most you can afford here, tweak away for everything else. As for tweaks, I've found purpose built audio components superior to the generic stuff. LPS for every single tweak that requires AC quality fiber conversion or fiber service great for noise reduction, don't cheap out on any of this. Finally timing, I've found master clocks are extremely critical for trying to achieve a continuous analog wave form such as found in analog souces. I personally will never purchase another dac without I2S input, or proof/evidence that dac master clock and power supply to that clock are of highest quality. 

Keep in mind this very important point. Because Ethernet cables, switches, converters and filters make a difference, the difference is not always positive. 
So either buy used or buy new with a solid return policy. 
Amazon is a good source for fiber optic conversion kits and switches. Take the minimalist approach 
Get a switch if you need it or something like the iFi LAN iSilencer that you can plug into your wall Ethernet port then use a very good Ethernet cable out of iFi to streamer. Just one example of a simple configuration. 

From a lower priced Ethernet cables I like Supra with Telegartner connectors. 
https://audiosensibility.com/blog/store-backup/#!/Supra-CAT-8-Ethernet-Cable/p/380224641

I use Wireworld Starlight 10 Twinax for router to switch and switch to Roon Nucleus One. These cables are very high quality and reasonably priced. 
Best cable, Audience Hidden Treasure, goes into streamer (built into my DAC). It’s a simple enough chain to avoid negatively impacting sound quality.