Sounds like you'll always have to do this little dance for any string you want to pass through, so I can definitely see how that could become quite annoying.

For not being able to combine serde-derive and cxx FFI on the same struct, there's a simple trick that can be used for many such situations:

struct CxxThingamabob { ... }

#[derive(Serialize, Deserialize)]
#[serde(transparent)]
struct SerializableCxxThingamabob(CxxThingamabob);

That just moves the Serde implementation to a different struct, so that you can choose which one you want by either wrapping or unwrapping it.