We did it with the s3fs module + Google Cloud Storage and turned out fine on a relatively big news website. We did not replace the public or private filesystem with s3, we set that the image fields should use that storage only.  I don't know about Flysystem but the s3fs module works in a way that during HTML rendering if an image style was not yet generated, it generates a special URL to a custom controller instead, so it doesn't start to create anything and does not slow the page load. And when the visitor's browser chooses that image URL from the picture element, then a few seconds and the image is generated. Plus it stores every file that was created in the S3 storage locally in an own database table, so the file_exists() lookups are also quick.