Finally got chance to run v5.0.5 today and can happily report a trouble-free experience! 
This will not go down well with those still having problems, I know, so allow me to comfort you with a couple of things that kind of broke.
I did experience the following error when trying to open the last line of the CSV file:
[warning]
Web page connection issue. Retrying last command.
[error]
ipcPromise: onAsk timeout 3000 for cmd "RUN_COMMAND", args "[object Object]"
The page itself displayed the following message:
Your file was not found It may have been moved or deleted.
ERR_FILE_NOT_FOUND
The page title (in tab) read:
chrome-extension://gcbalfbdmfieckjlnblleoemohcganoc/
Opening the CSV file in Kantu’s editor I could see the last line showing:
Which I believe explains the error. (Anyone know what the red dot means?)
However, I did specifically delete the last line of the file in another editor before running the macro (because it was empty), so I will look at that again next time.
Another lingering issue was the macro slowing down over time. I’m not so concerned about that at the moment - I’m just glad it ran without manual intervention and error-free.
One last thing to note, this was not my usual environment: instead of running it in Firefox, on a fairly new laptop, I used Chrome on my 3-year-old home desktop. I also believe the internet here is better quality than where I normally run the script.
Of course, it’s just one run. I will update again after another, but for me it’s a ray of sunshine… at last!
Thanks to @admin and the team; don’t stop.