UPDATE on this:

The solution was to NOT use this Lambda layer. Instead I had to package up Sharp in a zipped folder alongside my index.mjs file, and I was able to trigger it that way. There were also weird additional requirements such as, ensuring you install Sharp in the right way so that it's compatible with the AWS Lambda function code architecture you're writing in. It was a genuine slog to figure this out but if you're struggling to get that Lambda layer working, the fix for me was NOT using that Lambda layer.