Thanks for the response David!
I am using the latest version of Retrospect for Windows. To clarify what is happening (for any readers of this post), native compressed SQL Server backups are going to a network file share via the Ola Hallengren SQL Server Maintenance Solution: https://ola.hallengren.com/sql-server-backup.html. The backups are retained for 3 months. Previously we have been doing tape backups via an old version of Symantec Backup Exec and System Center DPM, we are retiring those systems in favor of Retrospect due to numerous issues. However, we are moving to a colocation facility and tape backups are no longer viable. Our current monthly Iron Mountain costs are in the $500 - $700 month range.
Looking at the options, Retrospect is not really viable as a cloud tape replacement at this point. It doesn't support AWS tape gateway or StarWind VTL. For this use case I am going to use RClone with the copy command (https://rclone.org/) on an encrypted remote site. This will achieve the objective which is to retain the SQL backups for at least one year. We are going to BackBlaze B2 and I can setup a lifecycle policy to delete old backups. The monthly cost is comparable to Iron Mountain and retrieval fees are also inline with Iron Mountain tape requests.