How can this be accomplished while providing the LEAST amount of access to the third party?

Explanations:

Allowing only specified IP addresses to access the S3 buckets does not effectively manage access for a dynamic and frequently changing third-party user list. It would require constant updates to the IP whitelist, increasing administrative overhead. Additionally, it does not grant access to users who might need to access the files from different locations or networks.

Creating an IAM role with access to the S3 bucket and granting console login permissions to the third party provides excessive access. The third party would have full console access to the AWS environment, which goes against the principle of least privilege, as they may access resources beyond the intended S3 bucket.

Creating pre-signed URLs allows for secure and temporary access to specific S3 files without needing to create an AWS account or provide broader access to the bucket. The URLs can be set to expire after a specified time, limiting access to only those files needed by the third party, thus maintaining the least amount of access necessary.

Having the third party sign up for an AWS account and granting cross-account access to the S3 bucket is overly complex and introduces significant overhead. It provides more access than necessary and requires managing cross-account roles, which can be cumbersome for a dynamic user list. It also goes against the least privilege principle, as it allows the third party access to the bucket rather than to specific files only.