Seems like what you're after for this particular backup is more file-sync and not file backup. There is no function in the product that will duplicate what you're looking for. That is, back up files only once and then back up only new files from then on. I cannot think of a way to avoid backing up changed files to solve your particular use case.

I assume you're not concerned about editing a photo and re-saving it under the same name.

The best you can do if you're not planning on changing the files in any way is to use a custom retention (set it in the backup plan) and do not select any of the retention options. That will allow you to keep all versions of all files forever since you are not versioning, you are not setting a backup delete based on time so old versions will never be deleted, and you are not deleting files from backup even if they are deleted on your local system, so once backed up they are in backup storage forever.

You should not use the Block-Level Backup option which backs up changes within files - as opposed to backing up the file in full each time it is modified. You're not planning on modifications anyway.

If nothing is ever modified, then keeping all versions for all time will not add you your backup storage and pretty much do what you need. If you ever experienced a big corruption that caused a lot of files to be backed up again, you would easily see this in the backup notification. At that time, you can stop the backup, replace the hard drive, and delete any backup versions not needed from the backup storage tab.

Use a file sync app instead such as SyncBackPro. Each time you sync you will get a window that tells you which files that will be moved to your S3 AWS and you can easily un-check specific files from being uploaded. Plus, the photos will be saved as native files and you will not need a specific backup app to restore the files.

https://www.2brightsparks.com/syncback/sbpro.html

Thank you both, I'll have a look at brightsparks as well.