Where to invest the most in the "digital chain"?

Since your streaming, look at the weak points of streaming before changing DAC’s or room treatments.  The SOtM cable 7u is great.  Adding an EtherRegen (needs an LPS and better yet a Master-clock also) to use as a “moat” and using the 7u between it and the streamer will give you a cleaner feed.  I have the 7u and also have the older ENO filter.  The 7u is better.  I have 3 switches and use the EE 8switch as the first buffer from the house Router’s noise.

USB, Ethernet or other Digital Cables are very influential to SQ.

I have an ifi Purifier, a dozen PS Audio Noise Harvesters and 3 Akiko Corelli’s.  I find the more noise suppression I add the sweeter the sound in terms of background blackness.  If you can demo a Corelli you might be very surprised at what it does.  The thing about noise is you don’t hear it until its gone, it’s not like distortion or jitter.

Jitter is the bane of digital and reclocking the USB and Ethernet is the starting point.

Depending on your DAC’s inputs using a DDC to avoid USB may be beneficial.

Keeping all SMPS’s or LED/fluorescent lights off for the electrical leg you audio is on will greatly reduce electrical noise.  Shutting down/ unplugging electrical equipment like computers, any SMPS’s in the house ( if you look you will find them in every room) can’t hurt either.

Keeping unused audio gear off like a phono pre amp or CD player.

Separating your digital and analog gear on dedicated lines keeps the digital noise out of the analog.

Having done some initial listening tests this evening between Wi-Fi and the very complicated (and expensive) Ethernet path my conclusion is that Wi-Fi is just as good as over £1k's worth of Ethernet toys.

I plan to do some more tests tomorrow but I'm very pleased everything is still returnable! Now that £1k can be redeployed into a new DAC to experiment with.

You can’t recover lost DATA after the source , so I think of the streamer as a modern day "Transport"

While this is true, it is not a reason to spend more on a streamer. Every streamer, every ethernet switch, every cable, passes bit perfect data unless you choose an option to somehow manipulate it with DSP. Streamers are computers. Computers do not lose data.

If you are hearing differences it is not because any data was lost

Jitter is the bane of digital and reclocking the USB and Ethernet is the starting point.

. Data is sent in packets. The DAC does not use the clock from the USB or the Ethernet so reclocking the Ethernet and USB has absolutely no bearing on any jitter in the DAC.

If you are hearing differences it is not because of jitter from the source

I believe the DAC makes the biggest difference. I’m using a Musician Pegasus DAC, which uses .005 ladder resistor, the same accuracy as the Denefrips Pontus II and Terminator. I’m using a SMSL SD-9 streamer wireless which every type of connection to a DAC possible, including I2S to a Musician Pegasus DAC. I’ve owned a Topping D90 MQA DAC and while it sounds good, the Pegasus sounds analog to me and has me putting up the Topping DAC for sale. The Pegasus costs 1100 USD, and is worth every dime. The SMSL streamer is 399 and decodes MQA.

It sounds like you are at the starting levels of hifi. There aren't that many dacs for £1000. A lot of the models audiophiles recommend cost $3k or more. I have decided to be happy with what I have and wait a few years before upgrading. Then I may be able to find a nice, used dac for a good price.

If you want something now the Okto Dac8 has gotten great reviews and it has a streamer built in. Could be a step up for you.