To test wether the fastboot partition is writable after updating via flashtool I have to admit that at first I attemped to only partially update the firmware (get the fastboot update without overwriting Sailfish OS) which only kinda worked: The partition was writable after the update and Sailfish OS still booted but it broke the Wifi and Modem so I must have overwritten a wrong partition. So I had to flash the full android update and then reflash Sailfish OS which fixed everything.