Thanks to those who gave their thoughts. I’m more familiar with the art of backing up on Windows.
There’s much information on the web on the various Linux directories, and a bit on how to backup.
What directories do I need to back up? has quite a variety of thoughts you can read, if you wish to. Specific situtations can change things, so you still need to think about what’s worth it in your case.
It’s pretty easy to reinstall the OS, and even the packages you install later. Harder to replace is the irreplaceable data that you or some process or program create, so it can take some thought there.
System configuration is probably somewhere in the middle. Lots of times a fresh start might be OK.
Even harder, and I’m surprised it gets asked rarely, is how to use the backup to reassemble things.
Home directory or Windows Documents are simple, but after that, it depends on what software did.
Unlike with an image backup, you can’t just slam the file backup onto the drive and have it all work. Sometimes having the details and files around is still useful as reference, as you plan the recovery.
Better yet is planning in advance, and of course testing the recovery plan to some extent is good…
There’s some happy medium between backing up too much, and regretting that you missed things.
I multiply backup my PC with a highly fine-tuned frequent backup plus some rare get-a-lot backups.