Hello
I use a mi band 7 with gadgetbridge. I have set it up for a long time so I won't remember all the details, but I can tell you I got the auth key by reading the app log files. I remeber typing the command exactly as in this guide did not return the auth key, but I found it just by scrolling through the log file and reading.
haQ Has this something to do with the sandboxed play services?
I dont think so, as I dont have play services installed but managed to set everything up correcly. I created an user profile exclusively for this, created the xiaomi account using a simplelogin email, installed the official app (whatever it is called), got the auth key and deleted the user profile in the end.