On the main Task, you can create a list field called “Subtasks” and make it of type Subtask. You don’t need to fiddle with Unique ID’s anywhere…you can just add “this subtask” to “this task.” If you go this route, you might not even have to have a specific Data Type for “Subtask.” They can all be tasks, but if you put a List of Tasks on another Task, you can visualize those as subtasks.
I see your issue. You aren’t making a change to a list of TaskLists…you’re only making a change to a single TaskList. You are updating a list within that single item, but in English, what you are currently saying is this:
“Find me all the TaskLists that exist and add this subtask to every one of them”
What you want to say is this:
“Get me the single, specific TaskList I want to update, and add this subtask to it.”