Strict aliasing and casting union pointers

I have looked around this site to try to figure out if my use of casting to different unions is violating strict aliasing or otherwise UB.

I have packets coming in on a serial line and I store/get them like:

union uart_data {
  struct {
    uint8_t start;
    uint8_t addr;
    uin16_t length;
    uint8_t data[];
  };
  uint8_t bytes[BUFFER_SIZE];
};

void store_byte(uint8_t byte) {
  uart_data->start = byte;
  /* and so on with the other named fields. */
}

uint8_t * get_buffer() {
  return uart_data->bytes;
}

My understanding is that this is, at least with GCC and GNU extensions an valid way to do type punning.

However, I then want to cast the return value from get_buffer() to a more specific type of packet that the uart doesn't need to know the details about.

union spec_pkt {
  struct {
    uint8_t start;
    uint8_t addr;
    uin16_t length;
    uint8_t command;
    uint8_t some_field;
    uint16_t data_length;
    uint8_t data[];
  };
  uint8_t bytes[BUFFER_SIZE];
};

void process(uint8_t *data) {
  union specific_pkt *pkt = (union specific_pkt *)data;
}

I recall having read somewhere that this is valid since I'm casting from a type that exists in the union but I can't find the source.

My rationale for doing this it this way is that I can have a uart driver that only needs to know about the lowest level details. I'm on an MCU so I only have access to pre-allocated buffers to data and this way I don't have to memcpy between buffers, wasting space. And in my application code I can handle the packet in a nicer way than:

uint8_t data[BUFFER_SIZE];

data[START_POS];
data[LEN_POS];
data[DATA_POS];

If this is violating the SA rule or is UB I'd love some alternatives to achieve the same.

I'm using GCC on a target that supports unaligned access and GCC allows type punning through unions.