I haven't read any of the long sun books but I feel like I understand that feeling. Fwiw, I first finished all 4 books and was only satisfied with the end of book 3. Thankfully there's so much meat between those endings that it kept me going.
That disappointment is what made me realize that I must have missed some important details and start an immediate reread.
Once I started over on the first book it was amazing to realize how the clues are just everywhere. I'm reading the exact same words, but this time they're hitting way harder.
Lol that's a new one for me. It's funny because it's true 🤣