[BrightGlow]

>You could strip out systemd and put in a custom init system that doesn't conform to the typical Unix daemon startup expectations.

I actually don't think you can do this, or would even want to. Systemd doesn't really do anything special beyond putting some tooling around various Linux features. I've seen a lot of other Linux inits and all of them have to conform to those expectations because that is how Unix and Linux are supposed to function, if the init doesn't do those jobs then the system doesn't work.

>You could write your own X11/wayland-like display server thing (like Android does)

Conceptually the Android window system is not really that different from Wayland. X11 is more the outlier. If you're building a new window system, you probably want to do things with a very similar approach to how Wayland and SurfaceFlinger work. No matter what you do there you're going to be constrained by the requirements of getting OpenGL and Vulkan apps to work.