Can I control the weight of a library loaded through a theme preprocess function?
Can you define your subtheme's library as extending or overriding your base theme's library. You'd have to do that in your subtheme's .info.yml. I think that should work. I think if you went this route you would just have a .css file in your subtheme that entends -- gets added whenever -- your basetheme's library is loaded. In this case you're override wouldn't be a "library" in your subtheme. Drupal should be smart enough to load your css file whenever the basetheme's library is loaded.