If you are wishing to migrate from Oracle to RDS PostgreSQL then the simplest way would be to use AWS DMS and AWS SCT(Schema Conversion Tool)
While using S3 and Glue is complex and there you need to deal with schema conversion manually and it will make your task more complex. So I would suggest using DMS/SCT it will work great.
You can have a look at DMS, which would help you to work ahead.
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