What about using the cloud as my alternate backup storage device?
Each word of that phrase becomes progressively uncomfortable with its implications:
- "Alternate" is a good idea, but "additional" is a better one. iCloud can definitely be an additional tool in your backup strategy.
- "Backup" is an obvious necessity. If you should lose a file stored locally, having it stored elsewhere is advantageous. But plenty of people inadvertently delete a document stored in iCloud and only in iCloud. What then? Good question.
- "Device" is obvious, but it's a storage device that is not yours. In reality it's many devices, but they're all elsewhere. What do you do should those devices should become unavailable for any number of reasons? Another good question.
Yes use iCloud. Just don't rely upon it exclusively as a backup strategy.
That would have the advantage of one backup on site, one off site.
An indisputable advantage. How about keeping one of your portable, inexpensive, easily replaced, disposable USB hard disk drives off site—a question that logically leads you to the conclusion that you really need a minimum of three independent, redundant backups, with at least one of them kept geographically distant from the others at all times. Sheep, wolf, hay.
On the one hand, I am not inclined to trust the cloud.
My trust in any Earthly thing is limited as well.
On the other, Time Machine backups are encrypted, aren't they? How secure is that?
Very. An encrypted TM backup is completely useless without its encryption password. Don't lose that password. It's also advantageous because when a backup drive fails (not if but when) all you need to do is throw it out. On the other hand if that drive should come into someone else's hands, an unencrypted backup can be used by anyone to restore a Mac that is not yours, in effect making it a duplicate of yours. Solution: encrypt.
I guess there is also the question of speed. If it takers a few hours to do a full backup via my USB port, it might take days to do a full backup to the cloud.
It takes longer for obvious reasons, but no matter whose backup service you use there really is no such thing as a "full backup to the cloud". Apple is careful not to refer to it in that manner, but if you look at other (usually, highly advertised) "cloud" backup services reading their fine print will also lead you to that same conclusion.
There is no substitute for Time Machine. There are plenty of backup strategies but nothing else is as integrated with macOS or is capable of conveying its features.