Cant get telemetry working on STM32 & IBus

Question

I'm working on small project based on STM32 and a custom board. I run into a wall trying to setup a working telemetry from STM to transmitter (FS-I6X). I wonder if anybody can guide me to the proper solution. The board uses single wire for receiving and transmitting data (half-duplex).

So far i made a properly working receiver driving 4 servos, so receiving data via IBus is working fine, as well as parsing GPS data.

My code below:

void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {
    if(huart->Instance == USART1) {
        if (ibus_read(&ibus_data_struct, ibus_channels, ibus_data[0]) == 0) {
            uint32_t currTick = HAL_GetTick();
            if (lastTelemetryTime + 10 <= currTick) {
                ibus_send_telemetry(huart);
                lastTelemetryTime = HAL_GetTick();
            }
            // serve servos

        }
        HAL_UART_Receive_IT(&huart1, ibus_data, ibus_data_size);
    }
    if(huart->Instance == USART3 || huart->Instance == USART4) {
        // GPS stuff...
    }
}

void ibus_send_telemetry(UART_HandleTypeDef *huart) {
    uint8_t payload[1];
    payload[0] = packet_type;
    uint8_t header[3];
    header[0] = 0x20;    // Start byte
    header[1] = 0x02;    // Payload length
    header[2] = packet_type;    // Packet type
    uint8_t checksum = ~(header[0] + header[1] + header[2] + payload[0]) & 0xFF;
    uint8_t packet[sizeof(header) + sizeof(payload) + sizeof(checksum)];
    memcpy(packet, header, sizeof(header));
    memcpy(packet + sizeof(header), payload, sizeof(payload));
    packet[sizeof(header) + sizeof(payload)] = checksum;
    while(HAL_HalfDuplex_EnableTransmitter(huart) != HAL_OK);
    HAL_UART_Transmit(huart, (uint8_t *) packet, sizeof(packet), 300);
    HAL_HalfDuplex_EnableReceiver(huart);
    // an act of desperation - packet_type is set to 0x00 
    // at the beginning and incrementing every loop to avoid wrong packet type and value
    packet_type = packet_type + 0x01;
}

Any help would be appreciated!

I've tried googling proper solution, without success. Have no idea, why it isn't working.