After much sweat, I have solved the issue. It may well be of interest to someone else in future so I will post what I found..
Full manual clean setup, piece by piece got it working with zero issues, very fast & snappy.
So I downloaded the config, got the backup, and used a merge tool to do a diff...
After going through every single line, the conclusion is... that I did not reset the 'network tweaks' after upgrading to 2.5 :(
[image: 1617922549233-1bc0a6a6-65b9-42ff-9104-5f670e85bf47-image.png]
Had I have noticed this earlier, I might have saved myself a world of pain! Of course, with so many settings in so many places, it is easy to be blind to oneself, more fool me though as I was confident that I had checked and reset them post-upgrade after I saw that they were not required any longer to maintain Gb throughput in v2.5.
Many thanks for your tips, in the end, we got there and I am very happy at last, as are the rest of the household ;)