I’d love to work on a tech stack that can be mastered in six months, with just as much understanding as someone who has been around it for five years.

An experienced developer won't need that amount of understanding to do 99% of the tasks at work, and for the remaining 1% there is documentation (right?). And if that's all your homebrewn stack, something is very wrong there.

There are very few companies that have a completely unique tech stack that couldn't have been encountered anywhere else.

Depends on how similar the domains are. Stacks are vastly different in companies working in disparate domains -- I've worked as a software engineer in C4ISR, gaming, EDA, oil & gas, HFT, remote sensing, consumer electronics, and more. The stacks couldn't be more different & foreign from each other. Like living in different universes. Six months to learn all the moving parts is about right.

(I can't imagine a career in only one domain; how boring would that be? Hopefully will be getting a new job in HPC soon.)

Your experience is probably atypical.

Documentation is one of those concepts that can be the answer to any problem, but seldom is.

I've yet to work at an organization where everything is documented, the documentation is up to date, and the documentation is of high quality.

There are always gaps somewhere in the documentation and in the end it does take time to ramp people up.

Now, if you work on project based work and it's always on something different, that's a different story.

Well yeah, the gp specifically said document the on boarding process. A high level architecture overview is all the system documentation required. Personally I ignore all that crap anyway, the truth is in the code and deployments. I don't know how many times I've had to tell the senior, long time employees how their system works.