SPISlave getting data without shift on the FIFO queue depth

I need to create a program for SPI slave which should send response appropriately depends on receive bytes.

I am using the following approach:

void recvCallback(uint8_t *data, size_t len)
{
    memcpy((uint8_t* )recvBuff, data, len);
    gLen = len;
    sentBack = true;
}

void sentCallback()
{
        if (recvBuff[0] != 0)
        {
            SPISlave1.setData((uint8_t* )recvBuff, gLen);
        }
        else
        {
            SPISlave1.setData(DUMMY, sizeof(DUMMY));
        }
}

But at first master got 8 dummies and only then starting the communication with the 8 bytes shift. As far as I understand I need to disable the bit TXMIS for SPI: SSPMIS Register (Datasheet p. 521)

Could you advise is it possible with SPISlave library? Or what is proper way with SPISlave to achieve response with 1 byte delay and not with the whole FIFO queue depth?

earlephilhower / arduino-pico

SPISlave getting data without shift on the FIFO queue depth #2314