On a Mac with an iCloud-synced Documents folder this is overly complicated. You should be able to run Publii on multiple Macs all reading the same ~/Document/Publii folder without any issue. Why does that not work?
Based on this, it seems I need to create backups for all my websites. Then move the ~/Documents/Publii folder somewhere else. Then install Publii on the other Mac and restore the websites from backups. I’ve modified the themes too. Will those changes be kept?