I am not sure if this should be on here or on the peripherals section of the forum.
I own a XPS 13 laptop and type c dock, I have erratic mouse movement and keyboard issues from these external components when used through the dock via the USB ports. If I leave the laptop slightly open the issues go away. Also if I plug them directly into the USB ports on the laptop I also get no issues. This is my current work around although for a brand new laptop and dock having to plug in 3 things rather than just the docking station doesn't make much sense.
I have tried the keyboard and mouse on other PC's with no issues.
Based on your troubleshooting, the culprit is the C/Dock Expansion Station dock. What are all of the Dell numbers on the dock so we can check for any driver updates for it? Also, have you tested the Dock on another laptop to verify that the issue is the Dock?
Are you using a wireless keyboard and mouse? If so, are you plugging the receiver into a USB 3.0 port? USB 3.0 is known to interfere with 2.4 GHz radio signals, which is why a common suggestion is to plug such receivers into USB 2.0 ports. Try that if you haven't already.