Turning off and on appeared to make no difference, so I went back to burner flashed to Orange/Teal screen, upload again. Same. This time got stuck on Teal screen.

In short, I think it was flashing correctly, but without anything on screen it is very hard to confirm state of device. I added a couple of Noddy lines to the boilerplate code – as per below – and ta-da.

#include "thingProperties.h"
#include <M5StickC.h>
...

And
void setup() {
M5.begin();
M5.Lcd.setRotation(3);
...

And
// Your code here
M5.Lcd.println("hello world"); // Print
delay(1000);
...

It just gives a heartbeat/health light style text as a visual clue of the device's state.