Ok, so I got it working! I decided to go with the Falken 1 ( https://www.thomann.de/gb/kse_falken_1.htm ( price: 112Euros/ 128 USD )). It has a 9V 2A output and someone in the reviews mentioned using it successfully for his Hx Stomp.
The Flylinktech jump starter mentioned above provides 9V 2A (not 3A) too, but I don't know, if a part of that 2A is somehow lost, when converting from USB to DC Jack. So I decided to go with the more expensive but guaranteed-to-work choice.
Oh and the Falken 1 includes one cable that has the right size (DC5.5x2.5mm) for the Hx Stomp's jack. The other included DC cables didn't fit, so I imagine that if someone decides to try out the method with the powerbank + seperately bought DC cable, then they would have to buy a cable with that specific DC5.5x2.5mm jack.
I briefly tested it at home yesterday. The battery depleted a little bit after 4 hours, which can be limiting, but works fine for me.
I was using all 6 blocks, the Hx Stomp got as hot as usual and there was no noise. It sounds great actually :)