Note that optimizing bulk loads with AWS RDS postgresql is somewhat different than using on-prem or self-hosted postgresql as you cannot directly login to the RDS host and using the local postgresql copy command.
Here are some very good references on the subject.
https://aws.amazon.com/blogs/database/optimized-bulk-loading-in-amazon-rds-for-postgresql/
Prerequisite setup:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PostgreSQL.S3Import.html
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.IAMPolicy.html