How to set an unattended Mac to automatically download files & photo originals from various iCloud accounts e.g Family Shared iCloud Storage?

Solution 1:

You could try enabling Fast User Switching, then logging in to all of your family’s accounts. Fast-switching between users keeps the previous user logged in, so processes running on that account should keep running in the background (albeit with a lower priority).

1. Open System Preferences → Users & Groups → Login Options.
2. Click the Padlock to unlock Administrator-only options.
3. Finally enable Show fast user switching menu as (Icon or Name doesn’t matter).
4. Now click the new option that should have appeared on the right side of your macOS menu bar, and login to another user account.
5. This should be sufficient since cloudd and photolibraryd will start automatically to sync your files and photos. However if you want to be safe, open up the Photos app.
6. Repeat steps 4-5 for every family member.

You should only need to repeat this procedure if you reboot your Mac (eg. after a system update).

Screenshots courtesy of How-To Geek.