Hi Jane - From what I understand, you'd like to have a single announcements forum to make posts which are relevant to students in multiple courses. And you'd like to make each post just once, instead of having to copy-paste it into the announcements forum of every course.
One approach might be to set up a separate course with just a forum, eg, a course made with the 'social format' which would show just a blog
-style forum on the homepage. You could even call this course 'Announcements'. Perhaps you could set it to force-subscribe or auto-subscribe to make sure your posts go out as email copies to everyone enrolled. And you might want to remove student posting permissions if this to be a pure broadcast forum.
Then use the course meta link feature to add your students in the different courses to the announcements course: https://docs.moodle.org/311/en/Course_meta_link
. You could also enrol them manually, but course meta link will be faster and more scalable. Finally, you might want to add a link from each course homepage to the announcements course.
Not sure if you would find this approach suitable... let me know what you think.