Assistance with CloudFormation and CloudWatch Agent

Working on some CloudFormation to install and configure the CloudWatch Agent. However, I'm having trouble writing this literal file that includes variables so I can use it for CloudWatch. Towards the bottom of the code, I need the ${aws:ImageId} written as is and not replaced by a value in CloudFormation.

I've tried removing !Sub, but it results in an empty string. I've tried using $!{aws:ImageId} and ${!aws:ImageId} but CloudFormation removes the $, resulting in a string with {aws:ImageId} or it results in a blank value.

Two other thoughts - save this on a public S3 bucket and pull it down or create a new image with the file and CloudWatch already installed. This is for a hands-on lab where I ONLY have access to CloudFormation and other AWS services,*** and the accounts are ephemeral.***

Running out of ideas, so I'm posting here for some help.

  Step03EC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t3.micro
      IamInstanceProfile:
        Ref: InstanceProfile
      ImageId:
        Ref: LatestAmiId
      SecurityGroupIds:
        - Ref: ProductionSecurityGroup
      SubnetId: 
        Ref: PrivateSubnetAz2
      UserData:
        Fn::Base64:
          !Sub |
            #!/bin/bash
            sudo hostname prd-app-cache-002
            yum update -y
            yum install -y amazon-cloudwatch-agent
            echo "Writing CloudWatch Agent configuration file"
            sudo cat << EOF > /opt/aws/amazon-cloudwatch-agent/etc/amazon-cloudwatch-agent.json
            {
              "agent": {
                "metrics_collection_interval": 10,
                "run_as_user": "cwagent"
              },
              "metrics": {
                "namespace": "prd-app-cache-002",
                "aggregation_dimensions": [["InstanceId"]],
                "append_dimensions": {
                  "ImageId": "${aws:ImageId}",
                  "InstanceId": "${aws:InstanceId}",
                  "InstanceType": "${aws:InstanceType}"
                },