Mirror Display Control allows you to disable mirroring by default when you connect additional displays. It is a comprehensive menu-bar application for macOS. It lets you control how second displays should behave, meaning you can configure that second displays are automatically not mirrored. Additionally, you can enforce mirroring also for displays which previously had mirroring disabled. And of course, you can toggle the current mirror state directly in the menu bar. In addition, Mirror Display Control can be configured to launch at login. You do not have to worry about newly attached displays. They are always behaving as you have configured them.