meaning every step of building the kernel, including the steps taken to build the C compiler toolchain, are produced by code that is simple enough to check for correctness and safety.

Full-source bootstrap isn’t about just the kernel, it affects every piece of software. With GUIX and Nix, every single package can be fully traced back to the bootstrap seed.

Though it should be noted that you do require a running Linux kernel on an x86 machine in order to bootstrap.

it is not quite to the point where it /just works/ on a lot of the computer hardware that I own.

Unless we get some serious money, effort and/or regulation w.r.t. OSS firmware, that will likely never be the case.
That has nothing to do with its technology though, that’s a political issue. GUIX is a GNU project and acts like proprietary software does not exist/is not a basic necessity in 2023.