Persona 5 Royal third semester: Maruki’s deadline
Reach rank 9 with Takuto Maruki, the Councillor confidant, by November 17. His final rank then happens through the story on November 18. This is the confidant requirement for the extra semester. You still need to avoid ending the game early through later story choices. Source: GameSpot's Royal guide.
This page names confidants and dates, but does not describe plot twists. It applies to Royal, not the original Persona 5.
Why is Maruki stuck at rank 5?
His later ranks have a story-date gate. Reaching rank 5 early does not mean you chose the wrong dialogue. Resume checking his availability from September 20 and reach rank 9 before his November deadline. Source: RPG Site's Councillor guide.
Do Akechi and Kasumi unlock the third semester?
Maruki is the confidant that unlocks the semester. Akechi and Kasumi have their own extra content, so a full-content playthrough should also plan for them. Do not confuse those additional goals with Maruki's unlock requirement. RPG Site explains the endings in detail, with spoilers.
Keep the deadline in your playthrough plan
P5 Tracker lets you record confidant ranks and review monthly goals. It saves your progress in your browser, with file export for backups or moving devices. The full app includes more character and dungeon information than this page.