Here is a progression chart that displays the required key items to advance. Spoilers ahead.
Your endgame goal is to enter into the music box which requires 6 music notes. Collecting those 6 music notes does require plenty of items that will be accessible as you play. The issue is finding out "what's the order of items you need to collect?". Well it's...... flexible but complicated. I have mapped out a chart on which items gain access to which next item.
Click to enlarge...
Notes of the Progression:
- Main Objective is to collect 6 music notes to enter the Music Box.
- Key of Hope is the only music note that does not require any prerequisite items after the linear quest.
- Claustro is the only phobekin that does not require any prerequisite items after the linear quest.
- Getting the Magic Firefly appears to be a long quest, but its shorter if you skip grabbing Astral Seed, Astral Tea Leaves, and Ordinary Wax Candle. Skipping those items would result in navigating the Dark Cave blind which is possible but risky. Ask the speedrunners how they do (its impressive).
- You do not need the Magic Seashell to get Lightfoot Tabi, because the maze is static. You would have to just memorize the maze itself.
- Lightfoot Tabi opens up many things including: the Pyro Phobekin, Key of Chaos, Sun Crest / Moon Crest, and the forward key items in the chart.
- Key of Courage requires the Demon Crown which requires rescuing 3 of the remaining phobekins. However the Key of Courage also needs the Magic Firefly for the Corrupted Future because without it, there won't be openings inside that level.