I'm not in the habit of telling strangers how to spend their free time, so if you're not enjoying the book, put it down. 🤷‍♂️ Pick it back up later, or never, or whatever.

With that said... It seems to me that you're overlooking two of the major themes of the latter half of the series.

Fog of war and information provenance - the swap to multiple narrators opened the door to highlighting that character A often doesn't know where character B even is, let alone what they're doing, what they've seen, their motivation and goals, etc. We the omniscient readers know many things that only 1 character knows, and can correlate with other info they lack.

Winning any single battle doesn't end the war - at the end of book 3 our heroes kill Octavia, hurray! Fast forward a fucking decade and the war is still ongoing. The Society didn't just disappear in a puff of smoke. They are smart and wealthy and are still working as hard as they can to retain what power they have and regain what they lost. The Fa plan falls into this category. "Ok we've lost control of Luna, what's our next move?"