Thanks for the input @vht981230. Seems with AWS CLI call (start-db-instance-automated-backups-replication) has similar inputs but this useful note:

Note: If you are using an Amazon Web Services SDK tool or the CLI, you can specify SourceRegion (or --source-region for the CLI) instead of specifying PreSignedUrl manually. Specifying SourceRegion autogenerates a presigned URL that is a valid request for the operation that can run in the source Amazon Web Services Region.

https://awscli.amazonaws.com/v2/documentation/api/latest/reference/rds/start-db-instance-automated-backups-replication.html

I ended up taking that approach in my pipeline with AWSShellScript task after my TF apply. Not ideal taking it out of the TF code...but easier than figuring out the pre-signed-url mechanism.