Hi Jedale,
I’m still sure there’s some specific app running on you MAC which try to access to your device then that is blocks the firmware updater.
Some user already reported following this method they where able to update the device properly: https://mac-how-to.wonderhowto.com/how-to/close-all-open-apps-your-mac-with-single-click-0160064/
First connect your device to your MAC, follow this method to close all your running app then start the updater.