I’ve enabled “Notify me upon replies by default:” under User CP > Board Preferences > Edit Posting Defaults, but I can’t see any way to get emails when replies are posted. Am I missing something glaringly obvious?

No, you are not missing anything. No one gets email notifications. At least, not the last time I checked. We’d all like them.

Usually, phpBB has an option to follow a topic, but either jps disabled it or this installation of phpBB is too old.