Thanks for the update and glad to hear it worked! 
Btw, I also just realised that I forgot a line in the proposed solution to register the DB: the database needs to be a Prodigy Database object as shown here so there's just one line missing that calls Database around it 
psql_db = PostgresqlExtDatabase(...)
db = Database(psql_db, "custom_postgres", "Custom PostgreSQL Database")
registry.databases("custom_postgres", func=db)