Dock firmware. If changing the cable did something, chances are the chips on the dock are not working nicely with the OS. It is a hit or a miss for what I've researched on it. What you have to know is that a Usb-C cable is only the physical shape of the port, what that port can do depends on ancillary chips implementing the protocols, and the protocol has to be supported on both sides of the connection. So, something on your current driver configuration is not talking with the dock. Maybe try running a newer kernel and see if that helps.
Also, the 30fps lock might be due to X11. I've not tried Wayland but this is one of the points the evangelists like to ramble about. Something, something, if both monitors are not equal and connected through the same protocol it doesn't work, just use Wayland or whatever.
For what is worth, lots of people do complain online about docks not working with Windows and some even stop working after a few months, those things are fickle as hell.