Why is it failing? Is it out of memory? If so flash with minimal.gz firmware then flash with the zigbee firmware .gz version disclaimer I have no experience of zigbee sonoff .but this is how you flash tasmota on esp8266 boards.
I tried the way you described. With the minimal release first and then with the zigbeebridge version. I had several problems. With the Tasmotizer I have often a “timed out waiting for packet” error, but the flash of the minimal release works always.
One time I could flash without errors the zigbeebridge version, but it was impossible to reach the board via WiFi (board not found with a IP scanner and no Tasmota access point).
So, now it works!
I repeated several times the identical procedure and suddenly it worked! Don’t ask me why, but this is very strange! I guess this is a timing problem with the FTDI interface.