Thank you. That does clarify things. So, if you wanted to do this, the steps would be,
0: Back everything up first.
1: Disable access to Nextcloud. (maintenance mode)
2: Move the data folder.
3: Edit the database table oc_storage. Change the local::/path/to/data to local::/new/path/to/data.
4: Edit the config.php datadirectory line to reflect the new path.
5: Turn off maintenance mode.
I get the impression this is not supported, but it should work?