Here, “a bit of everything” contrasts with the idea of “having it all”—so the writer is saying they’re content to have smaller parts of various things in their life, rather than striving to have (for example) a full-blown career and fully scheduled family life all at once.
An different, common usage of “a bit of everything” might be ordering food at a salad bar—the server asks what you want, and you answer “I’d like a bit of everything,” meaning you want some carrots, tomatoes, peppers, various other toppings, etc.