The backup verifies itself every night

Everyone has backups. Fewer people have backups that have ever been checked.

A nightly cron walks the ?simple listing of /critical: one filename per line, made for scripts. For each file it asks two questions over the same URL grammar: ?hash against local storage, and ?hash&backend=backend_s3 against the off-site copy. SHA-256 from both sides. Any mismatch, any missing file, and the job alerts before you've had coffee.

No backup software with a verification feature buried in a settings tab. A shell loop and two GET requests per file, because both copies are URLs.

A backup you haven't hashed is a hope, not a backup.