To me "C/C++, but secure" is equivalent to dreaming about sex on a beach. Sounds lovely as a concept, because you imagine the positives, not where the sand will get in.

It would be nice to keep the same language, same libraries, and codebases, and just somehow make them safe. But these were written for C++, not for a restrictive subset. You'll end up removing features that dependencies use (e.g. can you even ban non-smart pointers?). You'll add runtime checks to constructs that existing code assumed were free, and never needed to avoid (zero cost is a reason to use C++, otherwise a GC+JIT language can have runtime checks, but better). You will have to rewrite your code to clarify lifetimes, ownership, and remove constructs compiler can't analyze for safety.

You get many of the same pains that switching to Rust has, but none of Rust's features, and not even a fully-functional C++.