To first order, it doesn't matter which you pick, because we will all be better off than if you continue producing C code.
For production code, it is probably too early to switch to Rust, most places. Give it five years. But it won't be a mistake, then, to have five years' experience with it.
Rust and C++ will require more learning and adaptation than Go. Don't be fooled by C++'s apparent similarity to C: to get the benefits, you need to code the modern C++ way to reduce the window for low-level mistakes. Rust makes it hard to code the C way, so needs less coding discipline, but the Rust way takes more getting used to.
Both Rust and C++ can be substantially faster than C, for different reasons. C++ is better at capturing semantics in libraries. Rust knows more about what code is allowed to do, and can optimize accordingly.
Go is probably easier to switch to from C. There is not as much to learn. It's garbage-collected, so there are places it won't go, but most people won't need to go to those places.
It is never a mistake to know more than one language. In a sense, you don't have to choose one, you only have to choose what to learn first.