Enable cross-account management in AWS Backup, create a backup plan detailing the frequency and retention policy, tag the DB instances, apply the backup plan using these tags, and utilize AWS Backup for monitoring the backup status.

Activate cross-account management in Amazon RDS, establish a global snapshot policy with the specified frequency and retention, and monitor the backup status through the RDS console in the management account.

Initiate cross-account management in AWS CloudFormation, deploy a CloudFormation stack set from the management account containing an AWS Backup plan with the required frequency and retention, create an AWS Lambda function in the management account for monitoring backup status, and set up an Amazon EventBridge rule in each account to trigger the Lambda function on schedule.

Set up AWS Backup in each account, create an Amazon Data Lifecycle Manager policy with the specified frequency and retention, designate the DB instances as the target resource, and use the Amazon Data Lifecycle Manager console in each member account to oversee the backup status.