Secons Microvga dumb terminal.

That looks cool. I just ordered one so I might be able to help you further in a week or so, whenever it arrives. Meanwhile, their example sketch looks pretty strange to me. They seem to be using SPI "the hard way".

Read this:

I would be doing (in setup):

SPI.begin ();

And then to display stuff:

  // enable Slave Select
  digitalWrite(SS, LOW);    // SS is pin 10
  
  char c;

  // send test string
  for (const char * p = "Hello world!" ; c = *p; p++)
    SPI.transfer (c);

 // disable Slave Select
 digitalWrite(SS, HIGH);

However, untested until I get my hands on the hardware.