Multi-site vs Large Single site for Intranet
Another alternative you may want to consider is to start using the Group module, which allows for creating arbitrary collections of your content and users on your site, and grant access control permissions on those collections.

Group is available as of D7, and has a D8 version also. It is designed to be an alternative to Organic Groups. The Group module creates groups as entities, making them fully fieldable, extensible and exportable.

For more details about the various types of roles used by the Group module, refer to my answer to "What are the various roles supported by the Group module?".

And because Group also integrates with the Rules module, you could probably transform your workflows related rules toward Group equivalents.

Resources


Community documentation for the Group module.
Video tutorial "How to use the Group module in Drupal 8 to manage editorial access control to different areas of a web site".