Can I let editors decide where to publish a piece of content?

You can! The most convenient way for editors is to choose from a list of Taxonomy terms per content item. You can then use your Flow configurations to filter content by these terms, per site. Your taxonomy terms can either be a 1:1 representation of your sites (so every target site is one term) or you can group sites e.g. by specific topics or features to further simplify content management for your editors.