smickers

My issue [Array rebuild reaching 100% but never completing] turned out to be a partial faulty backplane in the Dell R720
 
As all my important data is backed up to the cloud, I bit the bullet and moved the Raid 0 drives from the PERC controller to the H200 HBA. I connected the H200 to the existing backplane, and boom, loads of smart errors, mostly CRC errors. I then connected the H200 directly to the disks and the CRC errors stopped.  FYI, the backplane is divided into two sections of 4 drives each, with one cable to each section; it was only the second section that had failed.
 
As expected, moving the drives from Raid 0 to a direct connection resulted in unmountable errors, so I followed the advice @JorgeB gave in other posts to rebuild each drive. Eventually everything rebuilt fine.  I'm now in a new Dell server with the H200 and currently running unbalance in order to move to the 18TB disks.
 
Two takeaways that may help others.
 
1. it's been mentioned several times and ignored by many (including myself) - don't do a raid 0 on each drive and pass through to Unraid - eventually this will catch up with you!  Seriously, don't do this. Unraid friendly HBAs are so cheap, just buy one.
2. Don't put your Dell server in the loft (in the UK anyway). Some of the casing - internal and external - has lightly oxidised, so I think the fluctuations in temperature and spells of humidity have upset some components. (It also has some fan redundancy errors that turned out to be the MB).
 
Thank you @JorgeB for your help here and your other posts.