Can I plan a whole saga before writing the first book?

Yes — that is exactly what the series-first flow is for. In your Library click "Start a series" (or "New Series" in the sidebar). Give the series a name, a premise, an optional book-by-book arc, and its writing style (genre, audience, style, tone, language, pen name). You land on the series workspace, where you can build the shared story bible — characters, world rules, your notes — before a single word exists. When you press "Write Book 1", the wizard opens pre-filled with the series style, and even the very first outline is generated already knowing your world, your characters and where the saga is going.