Certainly, use Announcements for any announcements you have, as long as it follows Rules for Announcements category, i.e. relevant and professional.
Announcements doesn’t either. Who said you have to update old posts with new info?
I don’t understand why you would need to do this. Just open a new thread, no need to delete anything.
I have no idea where you’re getting the idea that announcement threads have to be long. If you’re a plugin developer, create a new announcement whenever you announce something. If you’re a user, post a comment if you have questions/comments about the announcement. Simple as that, no need to make it more complicated than it is.
Announcements should be like reading a newspaper. Only the top post really matters, everything else is just chatter. For example, VCV Chords released doesn’t have too many comments, but it has 700 views. If you scatter your announcements at post #42 and #112 of an enormous thread, nobody’s going to see it.