I'm not sure it's reasonable to put time and effort into solving a
problem that C++ already has a feature that is designed specifically
to solve:

Modules.

Now yes, it's very unfortunate that C++20 did not standardize modules
for the standard library. But as a tool to reduce compile times due to
header inclusion, modules already work. Given appropriate compiler
support (which is admittedly thin on the ground, but you're suggesting
a change for C++23, so we should assume compilers and build tools for
C++20 are finished), `import <chrono>` should solve your problems.