Preface: I am a fan of accessibility overlays, especially when they can be used to reduce the usage of the mouse pointer to control you computer.
Now with the release of macOS Catalina, I think the new "grid overlay" and "numbers overlay" should be mapped to the keyboard as well. Imagine typing in Mail, then switching application and tabbing to the browser, and only with the keyboard, navigating the application!
How can I invoke macOS Catalina "Voice Control" features using only the keyboard?
How can I control my mac with keyboard only?