Understand the limitations of visualizing subtasks in a single browser list view. One possible way to visualize the subtasks in a single list view may be to use background colors on the row(s).
For example, the highest parent may have a different background color for columns star, actions, subtasks, and task. The child tasks for that parent could have a different background color for actions, subtasks, and task. The star column does not have a different background color for the child. This creates a visual indication of subtask and indention. Created a sample image that I could send if it would help.
The limit with the current list view would be 4 subtasks deep but creating a new, additional list view oriented toward subtasks may add additional columns for subtask visualization.
My use case is that I use Toodledo product for both work and personal. Personal tasks rarely require subtasks. Work /project/task management would greatly benefit from multiple levels of subtasks. For work, I mainly use Toodledo as a cloud repository for tasks and remote access from mobile devices. I use third party tools for detailed planning of the tasks.