Time Machine Stopping with No Error

Mojave 10.14.6, Mac Pro 5,1, 32GB DDR3, Sapphire Pulse Radeon RX 580 8GB.

I have Time Machine backing up to two drives – one internal and one external, both 3TB – which has been working perfectly until a few weeks ago. Both began  'Preparing backup…' as normal but then 'Stopping' after varying lengths of time with no error. Despite extensive troubleshooting it's still happening.

Here are the steps I've tried:

1. Running Disk Utility repair (several times) on startup Samsung 860 EVO 500GB SSD drive (TRIM enabled) and both Time Machine drives.
2. Running Onyx maintenance (several times).
3. Checking Console for errors/crashes = nothing obvious.
4. Making sure amount to be backed up is less than available space.
5. Trashing plists and restarting.
6. Adding startup SSD drive to Spotlight Privacy pane and then removing so Spotlight indexes again. 
7. Reinstalling 10.14.
8. Erasing one of the Time Machine drives, running Disk Utility First Aid and selecting as Time Machine drive = first backup completes, subsequent backups stop with no error.
9. Restarting from dedicated rescue SSD drive (Mojave 10.14) and running Disk Utility repair and Techtool Pro 11 on all drives.
10. Reinstalling 10.14 from rescue SSD .
11. Wiping startup drive, reinstalling 10.14 from rescue SSD, copying over backed up files (including very few ~/Library/Application Support files), reinstalling apps, re-erasing one Time Machine drive = first backup completes, subsequent backups stop with no error after varying lengths of time.
12. Excluding files copied over and ~/Library folder from backup in case corrupted files were copied over.
13. Running Etrecheck (report attached).

I'm back at square one – and I’ve lost six months worth of backups (although, thankfully, I did copy a selection of later backups). At this stage, if I had any hair, I'd be pulling it out in chunks. I just can't think of any other troubleshooting steps to take so any help would be very much appreciated.