I can't add a comment on @frankhermes' post.
Old thread I know, but it may help others.
You can easily allow designers to get latest submodule version by creating a MenuItem for Unity with editor scripts that either calls cmd with a fixed command through Process API, or integrate a .net git library that allows you to do the same.
Used the same concept for generating versioned packages in Unity when I was finished making changes for a newer version that updated package.json and created the archive I was supposed to upload to our internal npm server. Obviously custom packages might not have been a consideration back in 2018, but at the very least, Process API would still be accessible through editor scripts.