There are actually quite a few category ideas of a length between Darker Side and All Missions floating around:

• All Hats (560): Unlock and buy all hats. They cost around 4000 coins, and the last one unlocks at 560 moons
• All Achievements (601): What you said. One achievement requires 600 moons
• Darker Side No Moon Rock (503): Unlock and beat Darker Side without moon rock moons
• OG Moon list (~620): Collect all moons on each kingdom's original moon list. Excludes moon rocks and Toadette.
• 750 unique: Collect 750 moons without duplicates and add them to the odyssey.
• Any 999: Collect 999 moons including duplicates, then trigger the Really Lame Ending
• All non-achievements (819): Collect all moons, excluding achievements.
• Multimoons/Purples/Tracklist (MPT) (503): Collect all multimoons, all purple coins, and all tracks to listen to.