expdp Hangs at DATABASE_EXPORT/SYSTEM_PROCOBJACT/POST_SYSTEM_ACTIONS/PROCACT_SYSTEM in oracle 12c
So in the end it's just a time issue you have. You would have to open up a ticket with Oracle if you want to speed things up. This is probably not an Oracle issue, but possibly an issue with your database design.

You have to understand that the Data Pump Export Utility has to determine all relevant constraints and relations before it can create a valid structured metadata export. This can take a substantial amount of time and depends on the structure of your database, the amount of schemas and other limiting constraints.

The more complex the metadata is, the longer it can take to export.

You might want to look at the object after the .../POST_SYSTEM_ACTIONS/PROCACT_SYSTEM as that could actually be the limiting factor. 

You might want to have a look at the official documentation regarding Database Utilities | Data Pump Performance | Tuning Performance (Oracle 12c Documentation) for additional ideas on how to speed up things.