I admit that I was too scared to open some plugs at the engine, not knowing which one was the correct one on each side.
So I drained the radiator on the passenger side, which is very easy with gloves on and a quarter. The drain plug is actually staying in the radiator and you don't need to hold it or fish it out of the container. I used a large plastic pan to catch the roughly 7 liters of the 12.5 liter total of a 420 and disposed of it in the toilet. Press the hoses to get out more liquid.
Then I refilled with hot water and let the engine run until the thermostat opened. I also had the heat on so the heater core would get circulated through.
I did it a few times and then did the citric acid flush (15 minutes of 2500 engine rpms.)
Afterwards flushing with hot water again a few times (like 6 or 7 times. I calculated that I had the flush reduced to under 1 % of the volume). Then I filled it with MB antifreeze to get a 40% mix.
Yes, it took about 3 hours and my neighbors were wondering what I was doing...
Car runs at 86 to 92 now.