How to Parametrize Mappings and PermissionBoundary with aws cloudformation template effictively

I have below CFN template which is working fine, however as i am stiull learning and want my Mappings to be parametrized that's i'm not getting the way to do it. Secondly, the PermissionBoundary parameter i am not able to use it like Default: !Sub arn:aws:iam::${AWS::AccountId}:policy/CCoEPermissionBoundary as it probably doesn't like the !Sub function to be called and then referenced in the AWS::IAM::Role hence i am for now directly using it like `PermissionsBoundary: !Sub 'arn:aws:iam::${AWS::AccountId}:policy/CCoEPermissionBoundary' which indeed works well.

Can someone please help me on ..

1. get the Mapping to be parametrized and
2. How to to use PermissionBoundary parameter as a reference while using ${AWS::AccountId} in it.

below is the working code and commented portion is the one which doesn't work.

AWSTemplateFormatVersion: "2010-09-09"
Description: >
  This AWS Backup template deploys AWS backup-Plan for the FSx cloud resources.

Parameters:
  FsxIAMBackupRole:
    Type: String
    Default: 'test-fsx-backup-role'
    Description: 'IAM Role for FsxN backup Service.'

  FsxBackupVaultName:
    Type: String
    Default: 'test-fsx-backup-vault'
    Description: 'Provide the name of the backup-vault.'

  FsxBackupPlanName:
    Type: String
    Default: 'test-fsx-backup-plan'
    Description: 'Provide the name of the backup-plan.'

  FsxBackupRuleName:
    Type: String
    Default: 'test-fsx-backup-rule'
    Description: 'Provide the name of the backup-rule.'

  FsxBackupSelectionName:
    Type: String
    Default: 'test-fsx-backup-selection'
    Description: 'Provide the name of the backup-selection.'

  FsxBackupDeleteAfterDays:
    Type: Number
    Default: 22
    Description: 'Days to expire backups from vault.'

  FsxVaultMinRetentionDays:
    Type: Number
    Default: 21
    Description: 'Retention period in days that the vault retains backup data.'

  FsxVaultChangeableForDays:
    Type: Number
    Default: 3
    Description: 'Number of days before the vault lock. After this period, Vault Lock becomes immutable and cannot be changed or deleted.'

#  PermissionBoundary:
#    Type: String
#    #Default: !Sub arn:aws:iam::${AWS::AccountId}:policy/CCoEPermissionBoundary
#    Description: 'Provide Permission Boundary Name'
#
Mappings:
  RegionMap:
    us-east-1:
      schedulexpr: "cron(00 19 * * ? *)"
    us-west-1:
      schedulexpr: "cron(00 18 * * ? *)"
    eu-west-1:
      schedulexpr: "cron(00 17 * * ? *)"
    ap-southeast-1:
      schedulexpr: "cron(00 16 * * ? *)"
    ap-northeast-1:
      schedulexpr: "cron(00 15 * * ? *)"

Resources:
  FSxBackupIAMRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - backup.amazonaws.com
            Action:
              - 'sts:AssumeRole'
      Description: Create IAM role for backup service
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/service-role/AWSBackupServiceRolePolicyForBackup
        - arn:aws:iam::aws:policy/service-role/AWSBackupServiceRolePolicyForRestores
      Path: "/"
      PermissionsBoundary: !Sub 'arn:aws:iam::${AWS::AccountId}:policy/CCoEPermissionBoundary'
      RoleName: !Ref FsxIAMBackupRole

  FSxBackupsVault:
    Type: "AWS::Backup::BackupVault"
    Properties:
      BackupVaultName: !Ref FsxBackupVaultName

  FSxBackupPlan:
    Type: "AWS::Backup::BackupPlan"
    Properties:
      BackupPlan:
        BackupPlanName: !Ref FsxBackupPlanName
        BackupPlanRule:
          -
            RuleName: !Ref FsxBackupRuleName
            TargetBackupVault: !Ref FSxBackupsVault
            ScheduleExpression: !Ref FsxBackupScheduleExpression
            StartWindowMinutes: 240
            ScheduleExpression: !FindInMap
              - RegionMap
              - !Ref 'AWS::Region'
              - schedulexpr
            Lifecycle:
              DeleteAfterDays: !Ref FsxBackupDeleteAfterDays

  FsxTagBasedBackupSelection:
    Type: AWS::Backup::BackupSelection
    Properties:
      BackupPlanId:
        Fn::GetAtt:
          - FSxBackupPlan
          - BackupPlanId
      BackupSelection:
        IamRoleArn:
          Fn::GetAtt:
            - FSxBackupIAMRole
            - Arn
        Conditions:
          StringEquals:
           - ConditionKey: aws:ResourceTag/storage
             ConditionValue: backup-production
        Resources:
          - arn:aws:fsx:*
        SelectionName: !Ref FsxBackupSelectionName