I have now rather stopped experimenting, not that I still break the 120 euro display.
USB to serial TTL converters are available like sand on the beach and super cheap.
And many look almost exactly like the "original" from 4D.
I ordered one from Amazon and it will arrive on Monday.
It's so cheap that it doesn't hurt if it doesn't work.
Because the adapter from 4D has something that all the others don't have, the reset output.
And it is also triggered, for a hundred milliseconds.
If it turns out on Monday that the ordered adapter does not work with it, well then I have to buy the original.
I have made again parts list control, what do I have here, what am I still missing....
I soldered nothing yet, I will take the board with me to work and let the coils L1/L2 and the USB port soldering there because no way to do it here.
Oh, and the Heater board is now also ordered.