I am happy to report that my iPhone 7 update from iOS 13 PB7 to iOS 13.0 GM on a Mac went super smooth. Neither Apple developer’s account, nor any special software (Catalina or XCode) were required. Here is the update recipe, step by step:
1) downloaded the corresponding IPSW file from the links in this forum (a large 3,95 GB file, so it took me about 10-15 minutes);
2) connected the iPhone to the latest iTunes in macOS Mojave (unlike in previous iOS 13.0 betas, this time iTunes could actually “see” the iPhone, just as it did with iOS 12);
3) while holding “alt” button on the keyboard, clicked on “Check for Update” button and selected the downloaded IPSW file. At this point iTunes told me it needed to download an update, so I okayed it. Downloading the iTunes update took about a minute or two, then iTunes restarted in order to install the update.
4) I then reopened iTunes and repeated the ALT button + check for update step above. This time, with iTunes updated, it told me that iTunes will update the iPhone and verify the update with Apple.
5) The whole installation took about 7-10 minutes and the iPhone updated itself without any overheating, as it is often the case with OTA updates.
Now happily running iOS 13.0 build 17A577 and deleted the beta profile. Next “normal” iPhone update - to iOS 13.1 in early October. Meantime, will continue going through iOS 13.1 betas on my iPad Air 2, as there is nothing better for it to run until 30 September anyway.
Apple had quite a funky beta cycle for us this time round, so best of luck to all!