Several people have written that software. Duplicati, Borg, Arq for example.

> But then I’d have to monitor it, troubleshoot outages, maintain things, etc, etc.

None of these solutions free you from having to monitor your backups, including Tarsnap. Tarnsap requires setup on your server. You have to make sure it's running and backing up the correct files. And you really should verify you can restore a backup.

I'm really not sure what Tarsnap adds over these aside from saving you from having to sign up for B2 to S3 and punching in an API key.