Why do you need Kafka, Redis, Mongo, AND Postgres? Arguably it could all be done with Postgres or Redis: both have great pubsub implementations. But what could you possibly need two databases for and a queue implementation alongside a popular backend for queues?

I don't mean to criticize, but the complexity of this is what I'd expect for something handling hundreds if not thousands of qps. Even then I'd expect you could probably fine-tune a Redis Cluster setup and cross out 2/3 of that architecture diagram. Hell, you don't even need WebSockets: a system like this polling for an integer or two should be scalable to tens of thousands of qps.