Honestly though, I would likely still choose to implement the logic in Rust as a crate/library in a workspace, the (C)Python extensions as a PyO3 dylib/separate crate, and a C FFI dylib as yet another crate.

The benefit is that you only need to ship the CPython extension, you can build it with cargo, and everything's in one place. I've maintained bindings for Python and C# to native libraries, and it's usually a pain for various reasons.

The C API might be able to be used anywhere, but that doesn't mean it's ergonomic, easy to use, or safe.