I like how you describe it as "the two-stage nature of embedded development" because that is what in reality it really is. In my opinion replacing the preprocessor with another code generator wouldn't be the best path forward though. I have worked with a bunch of code generators and it has always been a crutch.

I think something that ties in with the target language and is aware of the syntax of the target language, like Rust macros, has many advantages. At the same time I do not know enough about Rust macros yet to tell if they are really powerful enough to help here.