I can confidently say the 'Wings of Fire' series by Tui T. Sutherland is a fantastic bridge between those two categories. The books are written at a reading level suitable for ages 8-12, but the rich world-building and complex themes make them appealing to older readers too. The vocabulary isn't overly simplistic, yet it avoids dense prose, making it accessible without feeling watered down.
The series tackles themes like war, loyalty, and identity, which add depth without overwhelming younger audiences. The chapters are well-paced, and the dialogue feels natural, keeping readers engaged. I've seen kids as young as seven enjoy them with some guidance, while teens and even adults appreciate the intricate plotlines. If you're familiar with 'Warrior Cats' or 'Percy Jackson', this series fits a similar niche—exciting enough for reluctant readers but layered for those seeking more.