With 4500 posts per minute you will probably get a lot of other scaling issues too, like with your database or the processing of incoming and outgoing activities. In any case its a good way to learn how Activitypub works. Is the code open source? Dont see it on your codeberg.

With the plugin system you basically just need a way to get notified about each new post and comment, right? I expect that will be one of the major use cases for plugins. We will likely provide various official plugins, eg push notifications for Android and iOS. The same thing should also work for you.Version 1.0 will let you subscribe to communities to get notifications for all new posts and comments (code).