Best practices for external backups for S3 buckets?
In terms of S3, for increased durability, consider cross region replication as an option if you want to increase the durability. AWS also provides their AWS Backup service you might investigate. I'm not certain of any benefit to transferring large amounts of data for durability to another provider. You'll pay a lot more for transfer, and it won't be integrated first class with the security IAM provides.

If the concern is about someone maliciously doing something then there are security best practices, access logging, cloudtrail, and other options to provide assurance. If it's more about the possibility (as low as it seems) for a bucket to fail, then the cross-region replication might be a great option for you with minimal configuration impact.