I actually just got it figured out: I read the release notes for Beta 5, which says "Specifying privacy-sensitive files and folders in a launchd property list might not work as expected and prevent the service from running." Not quite sure what "privacy-sensitive" files entails but I moved the script to the root of my Home folder instead of the Documents folder and it's working now. My Documents folder is synced with iCloud, so maybe it doesn't like that, or maybe it was getting hung up by the new Catalina restrictions on which apps can access what files.