cimt67 Thanks for the heads-up, I did not know about AVIF and it is just unbelievable. I tested with one image on my page, original PNG (with transparency) is 226kB, WebP is 34.7kB and AVIF is only 17.9kB. That is less than 8% of the original PNG!!
There is so far no good way to convert images to AVIF in PHP. Most of the solutions use specific libraries, but way outside of a scope of a plugin that should work on shared hosting servers and be basically plug-and-forget. So I will probably not add AVIF support to WebPConverter anytime soon.
But, maybe I can set up remote service on my Raspberry Pi and query that to convert images. This would require running the plugin in the background to not overload my tiny Pi (WebP converter just runs before serving the website, which includes converting the images before serving them; this is also not optimal, but it runs just fine for webpage on shared hosting with almost 100 images on a single page), but in principle once AVIF images are cached, you do not need to create them again. But this would require making a completely new plugin (with better suited name), so this will probably take a lot of time.