OK, seems I have managed to install /e/, so I’ll write down how, for others (and for my own memory, should I ever need to install it again). Just a reminder, that I’m not privy to how the system works and I was merely glueing together informations I’ve found. Also, I just hope that third-party site I got the needed file from did not modify it in any way, but I have no way to confirm it, other than it seems recognized on xda-developers.
Some steps were maybe not needed, but this is how I did it.
Following offcial instructions I encountered the same error as Mat35:
ERROR: This package requires firmware from an Android 8.1 based stock ROM build. Please upgrade firmware and retry!
E:Error in /sideload/package.zip (status 1)
My phone was on 8.1 and there was no OTA available. Following Mat35’s suggestion I’ve tried flashing “cedric-twrp-OPPS28.85-19-4-2.zip”, but the file I’ve found (on androidfilehost) was unflashable, adb sideload was immediately throwing an error.
On xda-developers I’ve found a suggestion to flash only modem firmware, taken from original Motorola ROM. I’ve found such ROM, matching my specific device (XT1676, RETEU — many posts suggest RETAIL, I suppose RETEU is for European version, and as the regulations concerning radios are different across the world, I wanted to have EU version) on https://motostockrom.com/ . In my case, it was https://motostockrom.com/motorola-moto-g5-xt1676 (the RETEU version is further down the page, not the first one. Check the filename).
From the downloaded .zip i extracted file Firmware/NON-HLOS.bin (md5 is 3605a87a6e4ba36c2f995204a7ec2aa6)
I rebooted into bootloader (adb reboot bootloader), issued command fastboot flash modem NON-HLOS.bin, then rebooted (still into the original system) to check that I can still access the mobile network and data. I could.
Then I followed the original instructions, with the additional step, that in “Installing a custom recovery using fastboot” section, just before flashing recovery, I did fastboot flash modem NON-HLOS.bin (yes, again, probably not needed), then proceeded to fastboot flash recovery ... and followed the instruction without further modifications.
Everything works, just one thing seems different: on stock ROM, I had “LTE+” mark, now it’s just “LTE”. I don’t know what the actual difference is; mobile data and calls work, and I have not tested speeds before, so I have nothing to compare against now.
I hope it would be helpful to someone who encouters the same strange error of ”updating 8.1 to 8.1”.