If there is no usable backup

Kustos rebuilds most of what it knows by asking your other applications. A fresh install pointed at the same Plex and the same arrs will find your library again, re-read watch history from the services that hold it, and re-index artwork sources. What does not come back is what only Kustos knew: your settings, your enrolled sets, your exclusions, your cleanup history.

So a lost database costs configuration, not your library.