Good to hear that you were able to recover the device.
The mt7621 has been changed to DSA switch architecture in 21.02. There is no guaranteed working migration path to take over existing switch config from older OpenWRT installations. If you upgrade an older version to 21.02 and do not wipe configuration, the switch sometimes does no longer work properly, as you have experienced. There must have been a warning displayed, recommending to not take over config.
A device, which is already flashed to OpenWRT, most likely will brick, if you try to flash a factory image via OpenWRT flash function. You must only flash a sysupgrade image via OpenWRT flash functions. There must have been a warning that you have chosen to override.
The initramfs image purposely does not persist configuration, to avoid wearing down the flash memory with many writes during testing. When you reboot an initramfs device, it will always start with OpenWRT factory default config (LAN on, WiFi off)