Thanks for your suggestion however, at this stage I am looking at a device that is not recognised by my computer.
My strategy is to firstly get my computer to recognise the device. Any suggestion is very welcome.
Check drivers in Windows of the need update. If you go to bootloader (fastboot) a driver is needed, if you later go to recovery a driver is needed. You might need to do it manually not use the auto installed.
Those are probably ok; we are only using a very small part of its capability, so just watch out for any error from the Command prompt output. The first test will come when you run
fastboot devices
You probably won’t get Windows to recognise the device as anything Windows can do business with as it stands in Fastboot mode.
Referring to the 4 part instructions from the install page …
bin-linux-x86 directory - which contains linux tools including fastboot and adb
[1]
You are now in a position to run
fastboot devices
This is the test to see that Windows does recognise the phone. [2]
Another test is to open a Device manager window, with the phone connected in Fastboot mode, does the phone show as a 
Problem device . If yes Windows update is expected to fix this.
Edit. It may be a good idea to keep a Device manager window open when you run the script flash_tetris_factory.sh – if the script should fail at any point check the phone has not newly become a Problem device – again Windows update is expected to fix this if a different driver has become required for fastbootd.
[1] To add clarity bin-linux-x86 directory this is platforn-tools, aka Android Debug Bridge, provided within the download. As long as you are in the directory as specified, Windows now has access to these tools from the command prompt.
[2] Should fastboot devices fail with “command not found” please move up a directory so that you are now in bin-linux-x86 and use
./fastboot devices
As showed in one of your post, the bootloader is already unlocked (by the /e/ installer).