Help needed with several engines use case
So any of those options EXCEPT ElasticSearch will probably work ok for you (equal hardware under the hood considered).
ElasticSearch is quite possibly the opposite use case from what you described. It's primarily best for tokenizing full text (like parsing email messages for example) and searching it efficiently, you mentioned your use case will always read by key.
A NoSQL database system or DynamoDB are actually rather fitting for your use case because of two things you mentioned: always reading by key, and storing the actual data in JSON format which allows for semi-structured, unstructured, and varying structured data. So this would be my first choice (might want to look into read limitations of DynamoDB vs other NoSQL database systems, as I thought I recall it was a little more limited based on how it was designed).
Otherwise MySQL should be able to handle your use case ok as well, being a modern RDBMS, but it wasn't really designed for your use case (serving the data only by the key and storing it all in a JSON format). It's a relational database system and therefore was designed for normalized schemas with Tables and relationships between those Tables.