This page https://docs.appimage.org/packaging-guide/converting-binary-packages/pkg2appimage.html gives some information about how to design an AppImage package and asks for suggestions.

I haven't heard much about AppImages and haven't found a repository of them, so I assume they aren't as well known as other package types and are at a disadvantage in gaining users because of that.
Different types of package probably have their advantages - for example containers, and with AppImages a single executable file. However different packaging methods (by which I mean for example a manifest.json and a meson.build file) not so much. I can't see any advantage in AppImage having its own packaging method.

I've published an app and I like the idea of the AppImage, so would like to create one, but I don't want to learn another packaging method.

ChatGPT has some ideas for converting existing packages into an AppImage but they don't do very much.

Have you considered using one or more of the existing packaging methods (eg a Flatpak manifest and meson.build file) to create AppImages directly, or at least have a utility to convert a Flatpak manifest to an AppImage definition file, or to convert from a Flatpak package to an AppImage? That way the author could produce more than one format with one definition. It would give you the opportunity of making more apps available as AppImages.