Meanwhile Rust, in this user's opinion, offers a superior toolset for modeling data and building behavior, namely: sum types and traits.
So if you're looking to extend or integrate with a C codebase, barring some exotic target platform and all else equal, I don't know why you'd opt for C++ over Rust.
where "as much work" means putting `extern "C"` in front of your functions and having a few `static_assert(is_pod_v<type_that_i_pass_in_argument_to_my_c_api>)` or only using opaque typedefs, ie it frankly amounts to nothing
If you are writing C++ in such a way that this does not matter, then you are basically writing C already and `extern C` is all you have to do though, but then you might as well just have been using C this whole time.
But when your API uses templates/generics and other types that C can't comprehend, you need to write additional wrapper functions operating behind opaque C types. It's not rocket science, and tooling/macros can help a lot, but it's still tedious and error prone (you're erasing types, redoing error handling, adding manual memory management, etc.).