This plugin does a great job of generating WebP variants of the images in the folder you point it at. If you're able to work completely outside of the WordPress media library and put just one copy of each image into the target folder then you may get good results. I don't know, unfortunately, since I've always used WP's built-in default mechanisms for uploading and generating multiple variants in the media library. Sadly this plugin has no ability to filter the folder contents and so it's useless to me.
On the positive side the developer responded almost instantly to my emailed questions to explain the problem which was very helpful.
My search continues but your mileage may vary.