efficient duplication of Time Machine backups to remote location
It is probably wiser to not backup the time machine backup, but instead make another backup of the computer directly.

The time machine format is a little finicky, as it uses a lot of tricks to save space while allowing to recover to many different dates, like reducing the amounts of dates one can go back to regularly. Even a minor corruption is likely to make it impossible to restore the backup of the backup to a functioning state. And that's exactly the situation you want the other backup for.

If instead you keep the backups separate, you have two completely different options to restore data if anything goes wrong, which should be useful in more situations.

Also, you can then use any encrypted backup solution you want.

Apart from that, here the official way to copy the time machine backup: https://support.apple.com/en-ie/HT202380

It suggests turning off time machine, then copying the backups.backupdb file to where you want it.

Note that this does not help with incremental changes, which is why I think it's not the best idea.

And here the closest to what you are looking for: https://www.berserkcomputers.co.uk/how-to-save-apple-time-machine-backups-to-cloud-services-like-onedrive/

In short, it suggests creating an encrypted image on your cloud drive with disk utility and then choose that as a backup destination.

Instead of copying the backup disk, just select two different disks, and time machine will use them alternatingly. This way, if one gets corrupted, the other should still work.

You can first copy the backup db to the image so that you keep the old backups, if you like. If your backup is already damaged, that will cause issues, though. If you have no likely need to restore an older version of your OS in the future, it's better to start a new backup, as the difference is mostly deleted files.