This feels kind of too-coupled to me.

In particular, there's a lack of symmetry between the handler (which just calls JX.invoke) and the renderer (which has a ton of application logic).

I think it would be cleaner to make this a default: case, call the handler, and pass it the message and the is_leader flag.

Then the handler would look like:

switch (message.type) {
  case 'aphlict.server':
    JX.Stratcom.invoke('aphlict-receive-message'); // maybe rename to aphlict-server-message?
    break;
  case 'notification.individual':
    JX.Stratcom.invoke('aphlict-notification-message');
    break;
}

Basically, handle protocol logic here but delegate all application logic to the handler callback.

With the adjustments above, we could also make is_leader available in these listeners, which would simplify them a bit (they wouldn't need to use a callback here).

This would become JX.Stratcom.listen('aphlict-notification-message', ...) or whatever.