Hello and thank you for sharing your experience !
I’m the fresh new owner of a pair of Moto G5 - Cedric devices, thank to the fact it’s a cheap one on second hand market (I’ve just buy one for eight €, yes, you read this right).
I have issue with the flashing process, because the OEM unlock just let you flash thing, but the recovery isn’t persistent.
The device I run test on might be of an older firmware version like in your case (I got the same This package requires firmware from an Android 8.1 based stock ROM build.), so your feedback is valuable to me.
However, the guide you point look buggy to me :
in the command to flash the firmware, there is
fastboot flash vendor vendor.img_sparsechunk.0
fastboot flash vendor vendor.img_sparsechunk.1
fastboot erase carrier
fastboot erase userdata
fastboot erase DDR
but the zip doesn’t contain any vendor.img
Further more, I’m no sure fastboot erase carrier is a command I want to run …
So I looked further through the internets and found out that the list of fastboot commands are specified in the flashfile.xml within the firmware zip (source : XDA).
Now i know what to do.
I hope this could be useful to others.
EDIT : it was for me, I now have a Motorola Moto G5 running /e/1.17 !