You would be able to steady shot at all times it in no way is tied to Aimed Shot. It’s the other way around, your Aimed Shot would be tied to Steady Shot.
It would absolutely be a mobility improvement. For single target you would only ever want to cast back to back Aimed Shots during Trueshot. Otherwise you would want to cast steady Shots between to setup massive Aimed Shots. You would see huge Aimed Shots when you stopped for them, and otherwise you would be fully mobile. You would always get Rapid Fire unless you never casted Aimed Shot.
Other than not wanting to Steady Shot, I cant help but think you didn’t even think about what you wrote.