> If I wanted working really close to hardware, do bit-twiddling and knowing where every byte is allocated, I'd take Zig

Why not C++? It allows as many low-level operations as one wishes, but don't forces you to manage memory manually where it isn't necessary.

> If I wanted to write a small piece of code intended to run absolutely everywhere

GCC and Clang have support of C++ since many years. Is there any modern platform for which no GCC or Clang backend exist?