Starmaker Story is built in a way where it is impossible to get all the content in one playthrough. The biggest example of this is with Anna, who has different content depending on which path you choose to take with the character. There is no way to access that content without another full playthrough.
Letting Mario into the relationship with Emma is one example of this, and it locks you out of some content and gives you access to other content. If you want to undo this choice you must reload a previous save or restart the game.
My advice as to "How to play Starmaker" is to do whatever decision feels right for your first playthrough, and then do a second playthrough later with the sole intent to unlock unseen content. Keep in mind new exclusive content is added with every update, so I am personally waiting until the final release to do a true completion run.
Don't worry about completion the first time through, the second playthrough is much faster than the first.