The idea is good. I would however propose a change (which might lead you to change even more): Have a look at my “reusable” shortcode repo here:
If you add it as theme component you will be able to maintain multiple repositories. The more to the right the repo is set, the higher it’s “importance” is. If you have for instance your repo, then mine, then your theme in a line and have an image code in yours, my image shortcode will overwrite yours. You could even overwrite more in the theme itself.
This is more useful than copy-pasting and hoping to not miss updates to your repo.
Just my 2 cents