To get that type of GFS (grandfather-father-son) retention, you'd have to move to the new backup format. However, the new backup format does not yet support hybrid backups, so you'd have to run two backups - local and cloud. The new backup format has some advantages over the legacy format: Client-side deduplication, synthetic full backups for image, file. VM, backup consistency checks, restore verification for image backups, Immutable backup option and GFS retention.

If you switched to the new backup format, you could set the following on the Retention section

• Set your schedule to run a full every 30 days and incrementals in-between
• Keep your backups for 90 Days (this is the last 3 fulls + associated incremental backups) - Now in order to keep 90, you'll need to keep 120 days before the earliest 30 days can be removed
• Enable GFS option
• Keep Monthly backups for 3 Years - there is no option to keep an end of year backup. You'd probably have to take a manual IBB on the last day of each year using a separate plan without a schedule and without any retention set
• You can Set Yearly to 7 years if you want though as that will retain annual full backups after the 3 years of Monthly backups

The Full backups will be synthetic on Amazon S3 (but not locally on the NAS). You also have the option to create an S3 Immutable bucket and send the backups to that new bucket. Then you can enable the Enable Immutability option (only available if the bucket is created with Allow Immutability - so you'll need a new bucket created).

Immutability uses Governance Mode and the Amazon S3 Object Lock feature to lock your monthly and annual backups according to your GFS settings for the plan.

I realize this is a lot to digest. If you have questions, please let me know.