efficient duplication of Time Machine backups to remote location

I have been using both Arq and Time Machine for backup for years, but a year or so ago I got unlimited Google Drive space and decided to try making a remote backup (with Arq) of my Time Machine drive.

This may seem ridiculous, but it helps ameliorate a major problem with Arq: you can’t move your remote backup from one service to another. For example if you signed up for Amazon’s drive when it was $ 5 a year for unlimited space, and used it for your Arq backups, and then decided not to pay $ 120 a year for sufficient space for your backups when Amazon said “oh, just kidding!” and changed their prices, you’re unable to move your backup history to another location. (Since history is the whole purpose of a backup system, this is the one reason I no longer recommend Arq to anyone. I used to enthusiastically recommend it to friends and family.) Backing up your Time Machine disk allows you to have a remote backup that you can move from one service to another without losing any history. (The only obvious downside is that you must first restore the whole Time Machine backup if your Time Machine disk has died and you want to change cloud provider at the same time.)

There is a non-obvious downside, however: Arq really sucks for this use case, for reasons I don’t understand. It gets through about 3400 GB (out of around 3600 GB) around the first day, then runs for days and days making virtually no progress, and then might at some point start to make some progress. I don’t know if I have ever seen it complete the backup. Many times I need to shut down my laptop before it completes.

I’d like to have an efficient way to backup my Time Machine drive to a remote service. It can’t depend on (e.g.) running rsync or Resilio Sync on the remote service.

In case it’s relevant for any reason, I have fiber, so the bandwidth is limited by the devices on both ends and any throttling the service does, rather than by the quantity of data pushed.)

One thing that Arq got right is the end-to-end encryption, so any replacement should include end-to-end encryption. I’m not about to trust anyone with everything.