You choose a class depending on your use case scenario and performance access requirements. You have to look at the access patterns for objects to determine which is the best storage class. For your use case, it sounds like you want to avoid any archive tier that would result in a long retrieval time. If you know the access patterns then you can organize your objects and put different lifecycle policies on the prefixes to tier them down at certain intervals.
You can also look at Intelligent Tiering which will handle this for you. S3 Intelligent-Tiering is an Amazon S3 storage class designed to optimize storage costs by automatically moving data to the most cost-effective access tier, without performance impact or operational overhead. It wont use any storage class that has a high retrieval time, like Glacier Deep Archive.
Since you have unpredictable access patterns, but are aware that some files may be accessed less frequently, I would recommend using S3 Intelligent-Tiering: https://aws.amazon.com/s3/storage-classes/intelligent-tiering/ . That should take care of the complexity of you figuring out which storage class to use for cost effectiveness, as it automatically moves between Standard, Infrequently Accessed, and Archive Instant Access based on frequency of access. You will also avoid delays when your users download files with those three storage classes.
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