There’s a new image format available that will result in smaller file sizes than WebP and it’s very flexible too. More info: https://reachlightspeed.com/blog/using-the-new-high-performance-avif-image-format-on-the-web-today/
Support for this new format is coming to Chrome and Firefox today. Maybe it’s a good idea to start looking into supporting this new format in Smush? How about an automatic conversion via the CDN similiar to what you’re doing for WebP now?