I guess I just bricked my MyCloud Mirror Gen 1

Did you use the correct firmware file? The single drive MyClouds have the information stored on the hard drive. This same (similar) system information for the two-drive MyClouds is totally different and is stored on the MyCloud hardware.

You will have to go through the entire process again with the files and instructions for the MyCloud Mirror and outline on Fox_Exe website:

https://fox-exe.ru/WDMyCloud/