If I got it correct, you would “only” need to switch to mailing list mode for that:
Update: … but that would ignore all notification settings, including the categories you’re subscribed to, which means you will receive EVERY SINGLE POST ON YOUR DISCOURSE INSTALLATION.
Updated update: It looks like that is only the case, if you don’t watch or track any category. When you do, you will only receive all messages for those regardless if they are watched, tracked or watched only first post.