XPS 13, WD19TBS, only running U3219Q at 30hz

I have recently purchased a Dell WD19TBS docking station with the intention of connecting to my laptop via a single cable.

I have the following computer and monitors:

1. 2019 Dell XPS 13 (i7-8565U + UHD Graphics 620)

2. Monitor 1: Dell U2719DC (2K, USB-C input + USB-C output for daisy chaining) 

3. Monitor 2: Dell U3219Q (4K, USB-C input)

My original plan was to daisy chain the U3219Q to the U2719DC which is in-turn connected to my laptop. This worked, however the U3219Q only ran at 30hz, which wasn't usable. I originally assumed this may be a result of the limitations of the integrated graphics on the XPS 13.

However, I then discovered that if I connect both displays directly to the laptop they would both run at native resolution and 60hz - exactly what I was after, albeit slightly annoying as it was two cables to connect / disconnect. My conclusion was that the integrated graphics was capable of running a 2K display + 4K display at 60hz, and the issue was in fact the limitations of daisy chaining. There seems to be mixed views on this across the web - lots of info saying that you can daisy chain 4k displays (albeit a bit of a grey area if this is both 4k and 60hz), yet a Dell employee on YouTube reckons you can't do this https://www.youtube.com/watch?v=6hx9jzs-gdQ. 

So I then did some research and it seemed like the solution to all of my problems was getting a docking station - WD19TBS which supports thunderbolt and should have all the juice required to run both displays at 60hz off one connection to the laptop. So I purchased one.

PROBLEM ONE:

However, if I connect both displays to the docking station, and then the docking station to the laptop via the single thunderbolt cable, I have the exact same problem  - the 4K one only runs at 30hz. This basically renders the docking station useless for my current setup.

Can someone please help me and identify what the problem is here?

Clearly the laptop can run both displays at both 60hz (I'm currently doing that via 2 cables), so there isn't a lack of power with the integrated graphics card.

Cleary the docking station should be able to run a 2k and 4k display both at 60hz - the WD19TBS should be able to eat that for breakfast.

I have two hypothesis:

1. I have a faulty unit of the WD19TBS

2. The capacity of the docking station is actually a function of both the docking station and the laptop its connected two i.e. the integrated graphics is an issue, but only in the situation it is connected to the docking station.

PROBLEM TWO

When I connect either display to the laptop via the docking station, non of the USB inputs work. Both the U2719DC and the U3219Q have a number of USB ports on both the sides and back of the displays. I have lots of computer peripherals plugged into these - mouse, keyboard, speakers, webcam, mic etc

Is is true that when you connect a display via a docking station the 'USB hub' functionality of the displays doesn't work? Or is this a specific issue with my setup as well?

I know they obvious response is why don't you just plug all your peripherals into the docking station - after all that's what its designed to do. the issue with this is that there is limited USB outlets on the docking station and I can't plug everything in.

CONCLUSION

Unless I have a faulty docking station, it seems that its actually pretty useless for my current setup - it won't run both displays at 60hz, and I lose all of the USB outlets on the displays.