Well it looks like I've bricked the phone. I downloaded the latest miui fastboot version, 8.4gb extracted,12.5.4.0, extracted it to a folder on my PC. I then booted the phone ito fastboot, connected it, got the windows jingle, found the device in miflash (latest version), loaded the rom, set miui to clean all, then started the flash. It was green, ran for a few seconds and then failed. I tried several times and nothing but failures. Changed usb cables, different usb ports, nothing worked. Since it failed I tried to reboot (mistake), nothing happened and the screen is black. I get the windows jingle when I plug the phone into a USB, but no device shows in This PC. Plugging it in and out in device manager does give a windows prompt, but no phone shown and can't find the device. Adb devices shows nothing.
Is there anything I can do with this phone to recover?
As I haven't tested any of this, best I can do is point you to my saved related resources:

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