After reading this post, I realised how worrysome the idea of losing my contacts would be for me. Although I back them up every now and then through the export function, it is far from systematic and I would probably lose a few newer contacts or updates if it ever happened to me.
If you use Nextcloud as server and Android on your phone, the Android Nexcloud app has a feature of making daily backups of the contacts. I haven’t tested a restore yet, although that would certainly be good to do before the worst-case scenario actually happens.
Then, if you have admin rights on your server, it is also possible to create backups of contacts (and calendars) through a cronjob, here is one description in German (haven’t tested that neither, but I have a daily backup of my entire Nextcloud database, which includes also the contacts).
Still, I would like the idea of having a separate Addon for Thunderbird to backup the contacts database on a regular basis. An Addon for the CardBook Addon so to say 😆 Anybody else?