I asked Gemini if it could create a shader for me - every attempt failed, so after about 10 iterations, I handed the project over to ChatGPT and it fixed it in 3 attempts.
If anyone's interest, it's here:
https://drive.google.com/file/d/1zE1s13RzZPtttzZrjiOLmEyAa1NL8Av_/view?usp=sharing
Three parameters - radius, width and height (you need to set the latter two to the size of the sprite).
Thanks for everyone's help.
That's an awesome result-now that's what I call putting AI to good use!
Not only did you solve the problem, but you also shared the solution with the community!