Hi @anon99228435, welcome to the forum!
This is a very good question! you can currently make a task/subtask a subtask of another task from the Advance actions in your task/subtask. However this action cannot be perform in bulk.
As a workaround, you can multi-select all the subtasks and drag and drop them in the main task list. Then you can drag and drag them separately to the subtask they need to be re-organized.
I hope this helps! Let me know if you have more questions!