Found a new image converter GUI app that can do batch #JPEGXL lossless conversion (rarest thing on earth, seriously?), but it's not on Flathub, it only provides Ubuntu and appimage packages, and the README says, "The recommended way of using XL Converter is through the official binary releases. The building process is time-consuming and tedious."
I don't expect it to be packaged for Fedora, so yeah, I guess I'm making a wish for an official #flatpak version here: https://github.com/JacobDev1/xl-converter/issues/95