To clean up My Tasks I created a rule that moves completed tasks to a new section called Done. However this rule doeswn’t work because we are working with subtasks. In the settings the subtask-option is grayed out.
But I just want to make sure you’re aware that a subtask assigned to you is not actually considered a subtask in the context of My Tasks; there it’s seen as a top-level task, so rules with this toggle off will still be triggered in such cases.
Finally, many of us don’t use “Done” sections and instead address this by adding a Done tab(s) with a Filter to show completed tasks only. Perhaps this could be an alternative for you, which many of us find more efficient.