For the critical data, using a warm pool1 can reduce the scale-out latency by having pre-initialized EC2 instances ready to serve the application traffic. Using On-Demand Instances can ensure that the instances are always available and not interrupted by Spot interruptions2.
For the noncritical data, using a second Auto Scaling group with Spot Instances can reduce the cost and leverage the unused capacity of EC23. Using a launch template with the CloudWatch agent4 can enable the collection of memory utilization metrics, which can be used to scale the group based on the memory demand. Adding the second group as a target group for the ALB and modifying the application to use two target groups can enable routing the traffic based on the data type.

https://aws.amazon.com/blogs/mt/automate-remediation-actions-for-amazon-ec2-notifications-and-beyond-using-ec2-systems-manager-automation-and-aws-health/

Enable AWS Config Across the Organization:
AWS Config provides a detailed view of the configuration of AWS resources in your AWS account. It can be used to assess, audit, and evaluate the configurations of your resources.
Enabling AWS Config across the organization ensures that all accounts are monitored for compliance.
Create a Conformance Pack Using the approved-amis-by-id AWS Config Managed Rule:
A conformance pack is a collection of AWS Config rules and remediation actions that can be easily deployed across an organization.
The approved-amis-by-id managed rule checks whether running instances are using approved AMIs.
Deploy the Conformance Pack Across the Organization:
Deploying the conformance pack across the organization ensures that all accounts adhere to the policy of using only approved AMIs.
The conformance pack can be deployed via the AWS Management Console, CLI, or SDKs.
Configure the Rule to Run the AWS-StopEC2Instance AWS Systems Manager Automation Runbook for Non-Compliant EC2 Instances:
The AWS-StopEC2Instance runbook can be configured to automatically stop any EC2 instances that are found to be non-compliant (i.e., not using approved AMIs).
This remediation action ensures that any unauthorized instances are promptly stopped, enforcing the policy without manual intervention.
By following these steps, the solution ensures that all EC2 instances across the organization use approved AMIs, and any non-compliant instances are remediated automatically.
Reference:
AWS Config Conformance Packs
AWS Config Managed Rules
AWS Systems Manager Automation Runbooks

Explanation
The following are the steps that the DevOps engineer should take to ensure that all EBS volumes always have the Backup_Frequency tag so that the company can perform backups at least weekly unless a different value is specified:
Set up AWS Config in the account.
Use a managed rule that returns a compliance failure for EC2::Volume resources that do not have a Backup Frequency tag applied.
Configure a remediation action that uses a custom AWS Systems Manager Automation runbook to apply the Backup_Frequency tag with a value of weekly.
The managed rule AWS::Config::EBSVolumesWithoutBackupTag will return a compliance failure for any EBS volume that does not have the Backup_Frequency tag applied. The remediation action will then use the Systems Manager Automation runbook to apply the Backup_Frequency tag with a value of weekly to the EBS volume.

Explanation
When AWS CodeDeploy performs an in-place deployment, it updates the instances with the new application revision one at a time, as specified by the deployment configuration codeDeployDefault.oneAtATime. If a lifecycle event hook, such as AfterInstall, fails during the deployment, CodeDeploy will attempt to roll back to the previous version on the affected instances. This is likely what happened with the two instances that still have the previous application revision deployed. The failure of the AfterInstall lifecycle event hook triggered the rollback mechanism, resulting in those instances reverting to the previous application revision.
References:
* AWS CodeDeploy documentation on redeployment and rollback procedures1.
* Stack Overflow discussions on re-deploying older revisions with AWS CodeDeploy2.
* AWS CLI reference guide for deploying a revision2.