Have you tried to disable the WiFi and Bluetooth using the DIP switches on the back of the phone before updating the ANX7688 firmware? You may also try disabling the modem before performing the update. You can always find the latest ANX7688 firmware blob in Ondrej's firmware repository.
Please note that some users reported issues with the ANX7688 firmware update when running Linux kernel 5.10, so you may want to use kernel 5.9; see also this thread for further information.
Please note that some users reported issues with the ANX7688 firmware update when running Linux kernel 5.10, so you may want to use kernel 5.9; see also this thread for further information.