Phabricator Tasks aren't trees, and aren't tied to an individual project or column.

A big issue here is about terminology: Phabricator doesn't actually have a "sub-task" object; The "Create Subtask" button creates a "regular" task, that's Blocking the "parent" task.

You can use the Advance Filters in the Workboard to HIDE all tasks that are blocking other tasks ("Subtasks"), or only show them.

Right, so I can make a parent task "Launch Product" and have subtasks of "Build Product", "Write Marketing", "Contact PR". All of these subtasks live on different workboards on different columns.

The two tasks most likely to approximate what you want are T4863 (indicate if a task is blocked or blocking others) and T10469 (pull up full task info without leaving workboard).