Aaaand now I got the SD2IDE working. I could install MS-DOS to it. But the system won't boot from the HDD. Same error as with the mechanical Conner drive.
And now with the new cable, I have coil whine when the system is under little load 😭 Have I borked the PSU, is the cable too crappy? I don't know. I'll have to investigate further when I have the time and motivation...