core_enrol_get_users_courses could be used to retrieve all the courses the user is enrolled on and at the same time detect those courses the users didn't access recently and are active (and not completed by the user).
With core_course_get_updates_since we can check if there are affecting the user

So with the combination of both, we can just notify the user that there is an activity in courses he didn't complete, have not finished and still is enrolled

We should also check completion and past courses as well? If course is completed or past for one user avoid sending a notification to him