I went through a similar "thought process" to build this: https://www.endbasic.dev/endbox.html last year. I originally wanted to "just" launch my binary right after the kernel started... but in the end settled for a full NetBSD base system to get things like network and WiFi configuration to work with ease. (That said, I still hook very early in the boot sequence to launch my own program and take over the console so that the rest of the system is invisible and initializes in the background.)