As it stands right now, it's not uncommon for authors to create addons for their own content, either as a customization or for experimental content/compatability bridges.

Unfortunately those are often uploaded on the same project, making it impossible to install the main project, and the add-on trough e.g. the CurseForge Launcher.

My suggestion to resolve this, would be a dedicated Add-On Tab for each project. Authors could see a "Create Add-On" button that allows them to create a new project, but also auto selects a newly added Add-On relation.

This would not only reduce the above mentioned issue, but would of course also benefit tons of other projects and provide an improved UX for end users.

E.g. in Minecraft where it's quite common to have addon mods for other mods.