The line that a proposal is a decision document, not a sales essay, is the right frame. One thing that helps me think about scope: add a short "assumptions" list right next to the out-of-scope list. Things like who supplies the content, how many revision rounds fit, and who gives feedback and by when. Out-of-scope items cover what you will not build, but assumptions cover what you expect the client to do, and that is where most brief changes actually start. If an assumption breaks, you can point to it and reprice calmly.