Writing a series teach consistency. Which is a good thing in writing. Be consistent with your character. I say this because I have read some stories where consistency was lost in the transition.
In writing a series the author make less mistakes with characters’ attributes and readers come to know the characters better.
Too many think consistency means boring and predictable. But it doesn’t have to be. Working with one main character’s attributes doesn’t have to be boring. You can grow your character. The character can live, grow, and change just like a real person. It can add depths to the story if the character to interact with many more characters than the single-plot where the character only interact with one or two others.
I realized adding more characters complicate the story but it can be done. How? Figure out the relation of the other characters to the main character before you insert them in the story. If they don’t fit into the scenario, then toss them or use them for a standalone book.
No, stories don’t always follow the writer’s planned format. They often grow and take on a life of their own. That’s the beauty of writing.