It's really difficult to figure out what is failing if STDOUT is not available from the post-processing script.
Simple file-based debugging has already confirmed script on v4 works to completion, but exact same thing in v5 fails to complete.
Here's a github issue that tracks this:
Simplify3D: Postprocessing script not executing on S3D v5 where it worked on v4 · Issue #4 · idcrook/s3d-thumbnail-generator-macos)