Sage isn’t set up to be a parent/child theme, it’s a starter theme you build from. I do agree that with Sage 9 especially being in active development, you will want to keep it updated. I would suggest adding the Sage repo as another git repo on your project, and then you can cherry pick commits from it. Something like this: https://gist.github.com/kalenjohnson/9ed257d8afb81e571a4b
Going off memory, something like this:
git add sage git@github.com:roots/sage.git
git fetch sage
git cherry-pick -Xsubtree=site/web/app/themes/sage 517be2a88318de51f51397066575ab07fae825a1 --strategy=recursive
Obviously you replace the sage name with your theme name, and you also update the commit checksum with the commit you want to add.
Then continue cherry picking commits in order from when you cloned to the latest commit until you’re up to date. And you can generally skip the merge commits as those are blank commits.