I have been using the "Find, Highlight, and Extract" action.
This tool works when I run it on smaller documents, but will usually crash or take a long time on larger documents.
For large documents 50k+ pages is there a faster/better way to extract pages with a particular phrase?
When you create a plugin for Adobe Acrobat it will be faster.
It's very unlikely that any script in Acrobat will be able to handle 50K+ pages. You would need either a plugin or a stand-alone tool for such a large file.