I have inherited a site with over 600 pages/subpages. I plan to use a plugin to change the way the navigation displays but it requires the pages to be in the Appearance -> Menu in this page:
I looked at other answers that were similar, and they were either old, incomplete, or non-programmatic. The solution I need should make it so the empty menu gets populated with all of the 600 pages/subpages. When that's done the plugin I'm using can style the menu in the way that I like.