C++ continue base class bitfield in derived class

Question

I have a base class with a bitfield

enum Enum {
 ENUM1,
 ENUM2,
 ...
 ENUM8
};

struct Base {
  Enum type : 3;
};

I am inheriting a class

struct Derived : public Base {
 bool bf1 :  1;
 u32  bf2 : 19,
 u32  bf1 :  9;
};

I need to ensure that the memory space for the Derived class is 4 bytes (I have a 32-bit microcontroller), since there will be several inherited classes and they need to be serialized, and when unpacking by the 'type' field, understand how to process the remaining bit fields .

Is this method reliable? If not, how can we do it better?

My CLang compiler does package everything correctly, but I'm not sure about the reliability of this result.