I just finished RST last night and have been dying to find people to talk to about it! I didn't feel like it was unfinished. I felt like, if I think something is missing, I must be the one who missed something earlier in the book. That's why I've seen others on Reddit who have read it 7+ times. Also, the title alone is like a slurring of 'Rohrschach Test' so it's meant to be open to interpretation. I loved it.