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)