At my university, students in groups of 1 or 2 write a fully preemptible, multiprocessor unixy kernel and userspace on x86. Essentially nothing is provided except a bootloader, a build system, a syscall spec, and some moderate protection from the details of some hardware beyond the CPU. This is considered doable in a semester by college students, some of who have no preparation except a general "here's what a CPU register is, here's what caching is" course. No one reads 20-30 books, but rather most students work from first principles and wind up reinventing the usual algorithms on their own.

I thought I'd offer a more optimistic counterpoint. I think that the 10,000 hours figure is way, way overestimating the amount of time needed in order to create something usable enough to make you satisfied, and which could teach you enough to understand any real operating system at the level of source code.

Although, yeah, if you've got commercial aspirations like OP, then I think you're in for it.