How to install iOS 13 on iPhone 6 OR how to use TestFlight on iOS 12.4.9 https://developer.apple.com/forums/thread/667749
No, you cannot install iOS 13 on iPhone 6 or 6 Plus. Max is iOS 12.
I see you answered to the very long thread in reference (and even reference it again in your post !). Unfortunately, repeating the question will not provide a solution. Did you contact support to see if there is a workaround ?