I am quite aware of weekends, holiday days and other planned time off. And I would expect a tool like Task Warrior to already incorporate all that - including being able to configure planned time off and configuring available work hours per work day. If that's not the case, then I'd say TW has a long way to go to become useful to the Project Managers I've worked with over many years.

Tasks, by whatever name they are called, should be arbitrarily subtask-able. A task with no subtasks would ideally be small enough to treat as either done or not. In reality, there are tasks that are large enough to need progress tracking, but are impractical to further subdivide. Or if are subdivisible, involve open-ended iteration to complete. Those tasks will be stuck with "airy" estimates. (The best tool for dealing with airy estimates is evidence based estimation.)

"Atomic Steps" are a convenience that can be simulated by "calculating" reasonable "default" values for the various fields. The simplest case being to insert special values that would not be accepted from the users. Otherwise, some fields would inherit the values from the parent task. Other fields can be derived. In one tracking system I used, it is possible to creating sibling tasks to the task currently being viewed. A new sibling could be either a predecessor or successor. Then, based on the effort estimate for each of the siblings, their due dates were automatically calculated from the parent's due date. And the effort estimate propagated to ancestral tasks, both to keep the ancestors updated and to generate warnings about potential schedule slip.

Anyway, good luck with your project.