Good morning Ian and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the struggle to change the text of the string in question.
I would love to help you with this topic.
I believe the reason the snippets might not have worked is because there is a variable in that string (There are no upcoming %s at this time.).
You could change it by changing the translation file directly OR by searching for the partial string (There are no upcoming) to see where the code is located and placing a filter on those function calls.
Let me know if that helps.
Have a great day!
Geoff B.