Restrict size of TimeMachine backups
I decided to return to plan A (copy Backups.backupdb), (having first made more swap space on my Mac by deleting movies) and closing down all running programs.

After more than 24 hours, this had completed the copy, and the files all looked OK.

I then enabled the disk in Time Machine, and ran a backup. This informed me it was making a new backup, and proceeded to copy 350GiB of data.

This was not quite as I hoped. Inspection of the file system indicated that none of the files were hard linked into the old backups.

As far as Time Machine is concerned, all my backups are there, and sample checks indicate all files seem OK.

Subsequent backups appear to working as normal, files are hard linked into the initial backup.

I did some pruning of earlier backups with tmutil delete, but it is obvious I am only going to get back to a 2TiB image with serious pruning.

I did toy with the idea of trying to combine the hard links, but decided I will leave the disk as is, and just accept that I now have a 3TiB Time Machine.
At least I do have all my historical backups if I need them .