Thanks @Alberto_Umana, Appreciate your response.
The use case I am trying to address is this: we are using Unity Catalog to manage access to our Databricks tables. Now that we are introducing some tables in Postgres DB, I want to control the read/write access to these Postgres tables through Unity Catalog as well. 
If I could control the access of the external tables in Unity Catalog, then I do not have implement separate security in AWS RDS Postgres using IAM. 

With the suggested approach of Spark/Pandas DataFrame, will it be able to leverage the Unity Catalog to manage the 'Write' access?