https://github.com/simulot/immich-go is what I used to import ~300gb of photos from Google Photos to Immich. Not sure how well it works for iCloud.
There is also https://ente.io/ which is a private & secure photo backup app that you don't need to self-host.
iCloud Photos stores originals (assuming you're using "Download Originals to this Mac") in the "originals" folder of your .photoslibrary package.
If you don't sync your originals, use iCloud Photos Downloader (https://github.com/icloud-photos-downloader/icloud_photos_do...) to get them.
https://github.com/simulot/immich-go is what I used to import ~300gb of photos from Google Photos to Immich. Not sure how well it works for iCloud.
There is also https://ente.io/ which is a private & secure photo backup app that you don't need to self-host.