Mobile device uniqueness for iOS and Android
From the posts you listed above, there're already answers to your question: on iOS, save a generated UUID into keychain, so that it persists even after re-installation. However, factory reset will clear that UUID, you should be aware of this limitation. This is not a foolproof solution, as it's limited by Apple's platform, but still, it's good enough and used by many apps.
One year ago I've written an article talking about a solution that tends to be "bullet proof" for iOS: https://blog.onliquid.com/persistent-device-unique-identifier-ios-keychain/ .
My solution uses Apple IFA or IFV, and then falls back to a random UUID if those IDs are not available. The unique ID that is chosen is locally stored on your NSUserDefaults and also on the Keychain (not synchronised to the iCloud, of course).
This way you can uninstall and re-install the app and the Device ID is kept the same.I'm using this approach in millions of devices without any issue ;)
Cheers,Miguel