That’s odd. I’m on the preview branch with the official dock, and 4k at 60 works, albeit with a lot of input lag for some reason. Have you checked the cable with another device?
What about 1080p at 120, can you just limit the fps to 60? You may even be able to use 40 fps at 120 Hz.
FWIW, the OLED should fare better in this regard as the screws now thread into metal rather than the plastic (and they have Torx heads too, which may help with getting correct tension).