building a community with different pages that all use the same page layout (aura component). however, with multiple pages using the same layout, when I edit one page, all of them are changed. is there a way to reuse the same page layout for multiple pages and override this behavior?
You can always set conditions to alter the behavior only for specific pages. At the moment, it is not clear what your page layout looks like (code wise).
Otherwise, you would have to create page layouts for groups of pages (multiple page layouts) which reflect the behavior you are describing, but will only do so for a group of pages.
Reusing the same page layout and dropping in different components shouldn't cause any problems with other pages. Did you mean theme layouts? If there are shared theme areas then it will affect each other.
If that is the case you could separate the theme to just be like navigation and footer then create a new page layout to use or use the Out of the box flexible layouts. If that's not an option then you could also just duplicate your theme layout and assign them to the pages but this wouldn't be the ideal option.