Should epics be named after big tasks/ Steps in user journey ? or Should they be just big features which can't be implemented in a Single Sprint?
So, now you might have epics that start, "as a user, I need a way to set up, maintain, and change my configuration so that..." as a start of one User Journey. It's just a way to help you think out the appropriate User Stories with which to provide context for what needs to be developed.
I really like Jeff Patton's book on the subject, "User Story Mapping" if you want to read more--available wherever you have access to Safari.
Agree with Ari Davidow's answer that User Story Mapping and User Journey's are powerful and useful tools for the team to understand the flow and better understand the MVP goal.
Epics/Initiatives should be defined and written at the highest User scenario possible where the Stories and tasks can then be broken down into the actionable work. The User Mapping would be a good fit at the Epic/Initiative level and the User Journey would be a good fit within the Stories.
Mountain Goat has a good article on the basics of User Stories: https://www.mountaingoatsoftware.com/agile/user-stories