[hobofan]

I've been using Amethyst for multiple years on only the builtin display as well.

I think the key here is to use multiple desktops (or are they called workspaces?). If I'm working on a single screen I constantly have 10 desktops open, with a quite consistent setup between them (e.g. first desktop is fullscreen browser, second screen is 3-5 tiled terminals, ...). All the applications I'm "actively" using are usually on the first 2-4 desktops with a lot of screen real estate for each of them.

What really helps here is assigning one of the screen corners as a hot corner that goes to Expose mode where you can switch between screens, as well as assigning shortcuts for switching between desktops and moving windows between them (the moving windows shortcuts are usually part of your window manager). Selecting "Reduced Motion" in the accessibility settings also helps by speeding up the desktop switching animations.