#!/bin/sh rsync --iconv=utf-8-mac,utf-8 -avh --delete-after --partial --progress /Volumes/myExternalDrive/Photos\ Library.photoslibrary [email protected]:"/srv/myExternalDriveBackup/"
(note: tested with brew rsync, IIRC the default rsync is outdated on macOS)
Somewhere in the directory structure is a folder /originals/ which has all the actual files.
Note that this is only a last resort backup. Restoring the library as a whole requires a Mac with a compatible OS version. Restoring without a Mac would only get you the originals, so only the out-of-camera files (jpg, heic, raw), with no edits or metadata changes from Apple Photos applied (Apple Photos doesn't touch the EXIF data). You'd probably also lose the video part of all live photos, as the live video files stored as separate files and not part of the .heic files. They're there, but not very usable.
An alternative to this workflow is to export all photos (with edits applied) from the Photos app, but honestly I'm not sure if that even works and how long it would take for multi-TB libraries.
There is a keep all files offline setting for iCloud Drive (turn off "Optimize Mac Storage" in Systems Settings).
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.