e.g.: Duplicacy Web Edition is missing on the Nix side of things.
It does exist on the AUR: https://aur.archlinux.org/packages/duplicacy-web
This keeps me from switching to NixOS :(.

According to the uninstall instructions here: https://github.com/probonopd/go-appimage/blob/master/src/appimaged/README.md#initial-setup

appimaged should create the everything itself in order for auto start to work after launching it once via ~/Applications/appimaged-*.AppImage

e.g. systemctl --user status appimaged.service says that the service is enabled for me.

(Maybe you were missing the --user flag?)

I would follow the installation instructions and if that does not work, the uninstall instructions in reverse to create the service yourself (probably with systemctl --user enable --now appimaged.service)

AppImageLauncher caused me problems in the past, similar to in this post.

So I switched to appimaged and have never looked back.

An implementation of AppImage tools written in Go by the inventor of the AppImage format.

After uninstalling AppImageLauncher, I had to make sure that ~/.config/systemd/user/default.target.wants/appimagelauncherd.service was also removed!

BTW the last release of AppImageLauncher is from 2020!