[shortishly]

You could use the proxy to spread topics over a number of brokers. The broker and proxy share a number of services and layers, that could be used to route:

https://blog.tansu.io/articles/route-layer-service

My itch for SQLite was smaller scale (and reproducible) environments, e.g., development, test/integration (with a single file to reset the environment). PostgreSQL was intended for "larger scale", with (database level) partitioning of Kafka records on each topic/partition, and replication for leader/follower setups, which might work better for HA. S3 for environments where latency is less of any issue (though with the SlateDB/S3 engine that might change).

S3: Not yet. I've been working through tuning each engine, S3 is next on the list.