I guess I misjudged Apple's commitment to support HFS+ long-term.
It now seems like there's no real native way of doing encrypted backups to HDDs on macOS (APFS not really recommended for HDDs)... the options are either encrypted Time Machine to a network location, or a third-party tool.
I posted ~7 months ago about my decision to use HFS+ for backups, and had a couple skeptical replies. (https://news.ycombinator.com/item?id=47317123)
I guess I misjudged Apple's commitment to support HFS+ long-term.
It now seems like there's no real native way of doing encrypted backups to HDDs on macOS (APFS not really recommended for HDDs)... the options are either encrypted Time Machine to a network location, or a third-party tool.
Guess I'll figure out SuperDuper or Carbon Copy.