[Moto G6 Questions & Answers] GedBlake:
Hi, Dan...

I really do wish I had some good news to impart, but sadly I suspect your Moto G6 Plus is probably not recoverable.

The OEM Unlocking setting in Developer Options is part of Googles Factory Reset Protection scheme. Ostensibly, it's designed to prevent lost or stolen devices from being reflashed back to stock and dishonest individuals then claiming the device as their own.

I ran afoul of this myself, four years ago, with my (then) new Nexus 9 tablet which originally shipped with Android 5, Lollipop and it was the first device I'd owned that required OEM Unlocking to be enabled in Developer Options before the bootloader could be unlocked.

I won't bore you with the details here, but if you have a mind to, you can read about my thwarted endeavours to resurrect my Nexus 9 here...

https://xdaforums.com/nexus-9/help/warning-locking-bootloader-disabling-t2951312

Fortunately, my Nexus 9 was still new, and I had a no-quibble warranty that I'd paid extra for, so I was able to get it replaced at the store from which I'd bought it.

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Around 2015 a solution (of sorts) was eventually found for this problem, for both the Nexus 6 phone and the Nexus 9 tablet, however it required a very specific set of criteria for it to work.

The OEM Unlocking setting in Developer Options flips a binary state flag, from either ON (Enabled) or OFF (Disabled) and it's stored in a special Persistent (PST) partition, which doesn't get touched when factory resetting the device, when receiving OTA updates or when reflashing it. As the name suggests, it 'persists'.

So it required somebody to create a dump image file of the PST partition with the OEM Unlocking flag already set to ENABLED on their device. This dump image file would be created by using a <dd, if, of> command in a terminal shell.

The person whose device was semi-bricked, would download this dump image file of the PST partition, and flash it to their device using a similar <dd, if, of> shell command, via ADB in TWRP Custom Recovery. This would overwrite their PST partition, where the OEM Unlocking flag is NOT enabled...

...with a version where it IS enabled, and consequently fastboot flash commands would then work.

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It required root on both devices, and TWRP custom recovery to be already flashed on the semi-bricked device, since ADB shell commands can not be run in the normal stock recovery.

You can read about the technical details here...

https://xdaforums.com/nexus-6/help/info-nexus-6-nexus-9-enable-oem-unlock-t3113539

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Theoretically, the method detailed above could perhaps be adapted for the Moto G6 Plus ~ you'd just need to find somebody with an already rooted Moto G6 Plus prepared to create a dump image file of their PST partition, with the OEM Unlocking setting enabled, and your G6 Plus would also need to be able to boot into TWRP to run the necessary ADB shell command to flash it.

And which is impossible, since (due to your locked bootloader) you can't either remotely boot TWRP from a laptop using fastboot boot twrp.img or flash it directly using fastboot flash recovery twrp.img

As I say, I wish I had better news for you.

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I hope all of the above makes sense; it is a little technical.

If I have any ideas that may be helpful, I'll post them here. But as it stands right now, and knowing what I know about this topic, I'm not optimistic there is a solution to your problem.

Ged.