I mostly use iCloud for photo sync. It would be nice if there was some sort of API or something that third party applications could use to upload and download photos automatically in the background, like iCloud Photos does.
[0] https://apps.apple.com/us/app/photosync-transfer-photos/id41...
https://github.com/icloud-photos-downloader/icloud_photos_do...
No uploading new photos, though.