How can I perform data validation using AWS DMS for a private RDS PostgreSQL database located in a different AWS account

You can peer VPCs across accounts. Peer the VPC in the account running DMS with the VPC in the other account.

Alternatively use transit gateway across accounts.

Thirdly you could also use private link and share just an NLB from the account with the RDS instance in which is targeting your Postgres instance.

Set up VPC peering between the source VPC (containing the RDS instance) and the target VPC (containing the DMS replication instance). This allows private communication between the VPCs across accounts and regions. Alternatively, use AWS Transit Gateway to connect the VPCs if you need to connect multiple VPCs or have more complex networking requirements.

Additional security considerations: -> Use AWS PrivateLink to provide private connectivity between AWS services without exposing traffic to the public internet. [+] https://docs.aws.amazon.com/vpc/latest/userguide/endpoint-services-overview.html -> Implement least privilege access by granting only the necessary permissions to IAM roles and users. -> Use AWS Key Management Service (KMS) to manage encryption keys for data at rest and in transit. [+] https://docs.aws.amazon.com/whitepapers/latest/logical-separation/encrypting-data-at-rest-and--in-transit.html

By following these steps and security best practices, you can securely validate data integrity between the private RDS PostgreSQL database and the restored backup while minimizing exposure of sensitive data and maintaining compliance with high-security standards.