> At 10k RPS

You would've used local memory first. At which point I cannot see getting to those request levels anymore

> ElastiCache Serverless (Redis/Memcached): Typical latency is 300–500 microseconds (sub-millisecond response)

Sure

> DynamoDB On-Demand: Typical latency is single-digit milliseconds (usually between 1–10 milliseconds for standard requests)

I know very little use cases where that difference is meaningful. Unless you have to do this many times sequentially in which case optimizing that would be much more interesting than a single read being .5 ms versus the typical 3 to 4 for dynamo (that last number is based on experience)

> At 10k RPS you'll see a significant cost savings with Redis over DynamoDB.

You need to be more specific than that. Depending on your read/write patterns and how much memory you need to allocate to Redis, back of the napkin calculations still point to the fact that Redis can still cost >$1k/month more than DynamoDB.

Did you actually do the math on what it costs to run Redis?