With 4 dimms you are just not supposed to reach 6000 - just luck if you can. Signal quality decreases with the number of dimms and therefore maximum speed gets lowered with 4 dimms.
The adapter you are looking for is juts non-existent.You need an adapter for USB+DP to USB-C.Take a look on this list and on the options, maybe you find something fitting like one of the VR cables or the WACOM stuff: https://dancharblog.wordpress.com...