I really appreciate your comprehensive suggestion. It definitely have opened up options for me. Among all your suggestions I think I would go with

- Some "classics" can be considered middle grade (warning: some of these reflect racist and colonialist 19th and early 20thC patterns of thought): Robinson Crusoe; Peter Pan; the Jungle Book; Alice in Wonderland; Black Beauty (not racist, it's about a horse); Dr. Doolittle; Just So Stories

- The Harry Potter series starts off at an advanced middle grade reading level and gradually increases to a young adult reading level

- Another thing that might be fun would be books of myths from various cultures. I remember reading a big book of Norse myths and another of Greek myths around age 8-9. You could probably start with a collection from a culture you're already familiar with, which could assist with understanding.

Also, thank you for the complement about my writing!