Streaming ZFS snapshots to a file and storing that in a remote of your choice is not the usual way of backing up ZFS data at all (it’s actually recommended against).
The usual way is ZFS send and receive to another ZFS server. But almost no cloud storage provider supports that, other than rsync.net (and zfs.rent), which starts with $60/month. So you have to set up and manage another ZFS server with secure remote access, PIA if you ask me.
It seems to work well. Am I carrying some unrecognised risk that I wouldn't be carrying if I were backing up into another ZFS filesystem? How does that risk balance against the risk of data loss on the backup server? i.e. I am pretty sure GCS won't lose my data, but not quite so sure about the other services you mention.
If your monolithic backup suffers single a bit error, it's unrecoverable. If you zfs recv, though, you can recover all pristine files. And if you zfs recv to a redundant filesystem (mirror, RAID-Z), you can periodically zfs scrub and recover from those errors completely.
I've been running ZFS continually since about 2009, currently using its replication as a major part of the disaster recovery strategy for my employer, and in the intervening time having been through a number of disk failures, controller failures, etc.
Just chiming in here to say that I endorse every aspect of the answer that 7e has given you.
Have you restored any data using this system? A backup isn't a backup unless you've practiced a whole cycle.
Snapshots are incremental, so management of your retention period is an active process and may require combining older snapshots together when you drop them.
With a zfs server on the other end you can zfs send / recv the other way to restore, and it correctly manages deleted snapshots.
They are but, unless the -i flag is specified (it isn't in my example above) then zfs send will send a complete replication stream, not an incremental.
ZFS receive checks the stream to ensure correctness. The stream can be easily corrupted, due to even a single error, if ZFS send is used without ZFS receive.
The error occurs in transit or on client side, not necessarily on remote.
Also managing a chain of incrementals can get unwieldy.
The usual way is ZFS send and receive to another ZFS server. But almost no cloud storage provider supports that, other than rsync.net (and zfs.rent), which starts with $60/month. So you have to set up and manage another ZFS server with secure remote access, PIA if you ask me.