While you can add to it - like you’ve already discovered - you can’t modify the existing contents of the <head> tag in a theme because it’s generated based on .erb files and themes don’t have access to that.
with a another file that’s loaded as a partial here
It’s possible that there maybe JavaScript workarounds to achieve what you’re trying to do. I might be able to advise if you can give me a simple example of the desired changes.