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. You're not planning on modifications anyway.

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

Brightsparks syncback