any way you could create a JCE plugin, which creates webp files and caches them on the fly - for inline image content?
Ideally, the plugin would catch any JPG & PNG images (in the frontend) and cache them on the fly. Somewhat like wf_responsive_widgets, which also works in the background.
YOOtheme Pro does this for builder pages, but not for images placed in Joomla content using JCE (which many of my clients use).
There is also DJ-Webp, which I have yet to try - but it would make sense to integrate this into JCE.