There are keyboard shortcuts to collapse bookmarks.
These shortcuts should be discoverable in the UI.
For example, the context menu for bookmarks should have the command, with the keyboard shortcut displayed next to it.

I had to do a google search to find the keyboard shortcuts explained in a user forum:
https://community.adobe.com/t5/acrobat/expand-collapse-all-the-bookmarks-in-a-pdf-file/td-p/2372796/page/3?page=1