Can lossless JPEG XL be used instead of PNG's for the source file? The jxl file doesn't have to be displayed on the site, more compatible formats can be used for that, but clicking the "Download" button would give you the jxl file. That way we can get the benefits of jpegxl without the compatibility problems (since stupid google claims there isn't enough interest in the format). Compatibility for devices themselves shouldn't be an issue since all major operating systems support jpegxl (including iphones and androids).
If this is done, please either allow only lossless jpegxl files, or tell us whether a file is lossy or lossless (like with a "meta" tag for example).