I discovered I can consistently get the screen to power on, in relation to my issue which I think is related to this one: Critical behaviour after suspension - #3 by Samuele
When the screen becomes totally black, I can lock the Plasma with a keyboard shortcut, then I can hit “Esc”: the screen flashes on, I can hit “Esc” again and it turns on. Clicking “Esc” again turns it off and so on. But this does not solve my problem: the screen shortly starts again to raise the brightness to the maximum and loop back to the minimum faster and faster until it becomes totally black.
The shortcuts for TTY for me do not work.