Not familiar with this particular library, but similar libraries use S3 Express One Zone which has write latency <10ms, so you can use that for the WAL and compaction can move data onto other storage classes in the background.
Regular S3 has write latency 100-150ms, which might be fine depending on your workload anyway.
You nailed it! With some of the similar products I've seen, they either inherit the Kafka protocol and hence the KRaft and other complexities, or go the other way with single-node only deployments, commonly just using SlateDB's single-writer model for durability.