(sorry if I miss smth, I kinda took the tl;dr approach).
Don't take the tl;dr approach. That's
exactly what Auto-Import's background scanning does, as long as your files are stored on a device that supports filesystem events.
It reacts
only to the files that changed, and it does handle deletions and updates to files (as long as those options are enabled).
The only reason you need to automate full scans is if you're using a NAS (often cheaper or older ones) that does not properly support filesystem events. If that is the case, you can automate the scans, but I'd consider getting a fixed storage device as well (sometimes this is fixed with more recent firmware, so check that first).
You can, if needed, also do one-off scans with all the same features of particular directories. This is covered in the regular Import article. But you have to set these up manually, and generally running Auto-Import will have the best experience.
