With some changes of the machine, arc frequency and duty cycle I could finally cut the 20mm aluminum bar, on which I failed miserably, yesterday. It took me almost an hour and around 1/3 of the wire spool, but I'm really happy to see that it's possible to cut such thick metal on a cheap desktop machine. I'm confident that I will find ways to further improve the machine over time to achieve faster cutting speed. In the footage, I cut the bar from outwards in, because for some strange reason the cutted parth got narrower as soon as the wire almost cut through the bar, so that the wire got pinched and broke up. I think doing it this way does also look better, because you can see the arc along the surface of the bar which would otherwise be inside the bar. The surface finish is also very smooth and you can see positions where the wire has broken up or the wire stayed in place for some time.
I think the next thing that I want to try is cutting some shapes.