Had a nightmare of a phone upgrade, and can't access quite a bit of data. Basically midway through the standard setup transfer process to a new phone, the original Xiaomi phone dropped out, and then apparently spontaneously changed its PIN, which means none of the Google restore methods work.

I had tried to complete the new install after the initial wifi file transfer stopped working, but after entering the old phone's PIN into the new one for verification, it said it was entered incorrectly. As my son hadn't altered the previous four-digit PIN in years, and I had in fact unlocked the old phone via that number only a few minutes previously, I was confused. Foolishly I then tried restarting both phones, only to find that the old phone agreed that the PIN had apparently spontaneously changed, and wouldn't proceed past the lock screen (fingerprint unlock was of course no use at initial boot).

After unsuccessfully trying a number of variations of the PIN and realising that the old phone wasn't linked to Xiaomi cloud which could have helped, I proceeded with a data wipe as I knew a standard Google backup of the original phone had been made only a day ago. However when I then tried to restore the old phone from its Google backup, that also required the unknown PIN as a final security measure (the never-changed PIN that was definitely correct when the backup had been made is apparently not the correct option). An installation attempt on the new phone from that backup also required the PIN. After another few attempts at guessing a random 4-digit pin, I was informed that I had only five more attempts before my Google backup was deleted.

I have ended up completing a clean install on the new phone, and although luckily I had made a backup of all the photos relatively recently, there's still a lot of data lost, including text messages and various Notes etc.

I have no idea why the original PIN stopped working (as it was midway through the transfer process I tried both the default OPPO and Xiaomi PINs, as well as some other obvious defaults and all variations of the original PIN before giving up and wiping the phone), but as I still have the original phone and access to the associated Google account where the backup image has been saved, I would have thought I should be able to restore it, but have run out of ideas.

Any suggestions on how to recover the image without using up my remaining PIN attempts?