Building a presence system: who's online, and how to know at scale?

we're looking to build a slack-like presence system to show which users are currently online. the main challenges are scaling to 500k+ concurrent users and handling all the edge cases. specifically, how do you deal with multiple tabs or devices from the same user? or flaky mobile connections that cause users to flicker between online/offline states? then there's the challenge of propagating presence changes efficiently to potentially millions of other users. we're considering redis pub/sub or kafka for broadcasting updates, and a database like cassandra or scylladb for storing the current state. what's the recommended architecture for a system like this, particularly around consistency, scalability, and handling the common user experience glitches?