Joseph, the point you made about allowing “the first draft to suck” cannot be emphasized enough. For all the novels, short stories, and screenplays which writers finish, there are exponentially more — I figure — which do not because of one simple fact: The writer get frustrated at the experience of writing and quits. That negative experience is influenced by numerous factors: they can’t ‘find’ the plot and get lost; the quality of the writing feels poor; Perfectionism and/or Procrastination hold sway.
By embracing the fact your first draft will *not* be perfect, that it will in all likelihood suck, that actually takes the pressure *off* you. It’s the very spirit of the ‘zero draft’ approach. Get the first draft done and see what you’ve got.
Screenwriting is an iterative process. You write a draft. You rewrite it. You rewrite it again. Each time, hopefully getting the story better and better.