It's been decades since "writing a 1000 line script" was best done in shell, if there ever was a time. Someone with that much experience ought to be a very senior engineer.
I have shell libraries that generate code via here documents and other templating that are > 500 lines. That is about as far as I reasonably go. Note that these are function (and scope specific variables (with hopefully unique namespaces)) only code bases with copious comments and references to underlying tools.