I can’t speak for Jenny Bryan, of course, but I’ve understood her sequential file numbering advice to be an intentionally simple (and therefore accessible) solution meant to cover relatively simple cases (which is a lot of cases!).
Once you have enough steps to your process where you’re worried about inserting data cleaning step 3a in the middle of the chain, I think it’s time to consider more formal pipeline management tools, whether that’s old-school make, shiny new drake or remake, or something else.