I also manage to flash now using Ubuntu, and the solution is VERY STUPID: I used the very cheap USB-A 2.0 to USB-C cable, and don’t use anything in between. Then I just follow the official guide and everything seems fine. I used USB-A 3.0 to USB-C cable before (swapped from my USB HDD) and that won’t work. Having USB-C extension cable also won’t work.