alt Hacker NewsI run a separate Mac Mini that has the full iCloud Photos library on a massive external drive, set to "Download originals". I then rsync that filesystem to a separate Linux box. This works but you must not ever disconnect the external drive.
I don't have a solution for iCloud Drive, as there wasn't a keep offline setting last time I checked. So use it only ephemerally.
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At least as of Sequoia, the Settings > iCloud > Drive > Optimize Mac Storage option enables iCloud Drive files to be stored offline. Likewise, right clicking any iCloud Drive files in the Finder includes a Keep Downloaded option. Since I minimally use iCloud Drive, in the past (older OSes) I also had Hazel make copies of iCloud Drive files so they were certain to be in backups.
I'm not familiar with the "Photos Library.app", but I have an m4 mini with my photos in a Photo's Library. I'd love to know your script to rsync the photos into a separate drive/directory
For iCloud Drive have a look at rclone. You can run it straight from your Linux machine
Arq [1] has an option to "materialize" dataless files, basically forcing them to be locally available. The only issue is if it's a large file and it gets pushed off device often, you can burn a lot of bandwidth re-downloading it over and over again.
1. https://www.arqbackup.com