Using Multiple Models For Multiple Tasks - #12 by sergeliatko explains why it’s better to split tasks as deep as possible.
Also I see in your JSON non printable characters and math formulas, you need to be sure about your character encoding in training JSON files to be UTF-8 so that it wouldn’t break. That might be the cause of the errors with your files.
Seeing your JSON and structure, I would personally try to isolate entities you’re operating with and track their sources. Here is my logic behind it (again, very subjective)
Your goal is to TEACH, STUDENTS some CONCEPTS which are broken into complex SUBJECTS with UNITS and LESSONS as children of first and second level, where TEACH means transfer UNDERSTANDING of the SUBJECT that can be verified by high SCORE obtained during the TEST…
I would start with building the CONCEPT object structure to understand what and how it can be taught…