I have several tools to open and edit packages, but they all only do one at a time. This is impractical when you have thousands of items to edit. I'm trying to find out how to open packages myself so that I can write a script or program to edit things in bulk. I just can't find any useful information on how to do this. Everything I find is ancient, or vague. Is there any instruction anywhere on how to open packages?

That simply just isn't how it works, unless you merge all of the packages beforehand- but there is no way to undo that process afterwards.
Scripts and programs can be written to affect the contents of folders, however. Sims4Studio has a whole array of batch fixes built in that run on any number of files at a time and if you're already familiar with writing batch scripts, it's probably quite straightforward to sample from the existing ones to make your own.

A batch fix is exactly what I want. But S4S only handles a few things, nothing like I want to do. You're saying I can write my own batch fix for S4S?