Does the S3 extension not have a ListObjects call? I thought that ListBucket served that role but when I look closer there's no filtering prefix to "only list the files in this metaphorical folder".

Remember, S3 does not actually have a folder structure. It's a key/value pair, with the name allowing slashes in it to kinda be like folders. You will probably have to do your own filtering.

I cannot "do my own filtering" on a bucket with tens of millions of keys. https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/s3/model/ListObjectsRequest.html#prefix() discusses the tool that answers that issue.

i haven't tried it myself, but the AWS-CFML extension does appear to have a list objects in a bucket with optional prefix filter https://github.com/jcberquist/aws-cfml