> Doing backups and validating backups is very error-prone and time consuming

Right, but regardless of using a managed database service or self-hosted database, this is something you probably are doing anyways, at least for a production service with real users. Sure, the managed service probably helps with you with the details of how the backup is made, where it's stored, and how the restore process happens, but you still need to validate your backups and the rest, so replicate that process/experience with your self-hosted setup and you're nearly there.