EDIT:
This is now available in Flowsana - you can have any of its rules trigger for just top-level tasks, just subtasks, or both!
Hi @Karla_Baaklini!
At the moment Flowsana has the same limitations that Asana does in terms of its rule triggers not operating on subtasks.
We have done some experimenting and I believe will be able to support triggers on subtasks, but more work needs to be done before that’s something that can be released.