Can i use the micro USB on the back of the device to flash my own software to Growcube?
A dedicated 5-pin USB cable is required to flash the software from this interface.
The cable below should work; you may consider purchasing it:
https://www.elecrow.com/usb-otg-host-cable-micro-b-otg-a-male-to-a-female-p-1543.html
In addition, you will need to use a USB cable with USB-A connectors on both ends (this cable needs to be able to transmit data).
Additionally, flashing the ESPHOME firmware is something you'll have to try and test yourself; we're sorry, we don't have the relevant information.
Sorry for the inconvenience.