Specced the heavens, max memory, ECC, Xeon.
I also had issues with the dock, but it was exclusively the dock, the TB16 is a flaming hunk of garbage that has no business being on my desk.
The issues mostly stem from the fact that the USB controller (which powers nearly everything in the dock) is shockingly terrible.
But, regardless, there are more issues because the 5520 (and probably 5530) have only 2PCIe lanes to the thunderbolt port. I was surprised to learn that "thunderbolt3" != "thunderbolt3" in all cases.
I have a 5k laptop that is a buggy pile of trash because someone tried to save a dollar on a USB controller.
Honestly if I could lug this thing at Michael Dells' head I would do so I'm so frustrated.
But anyway, the laptop by itself is fantastic, it does get toasty under full load but I tend to disable the GPU so it's bearable. I considered getting the new WD19 thunderbolt dock before I realised about the thunderbolt PCIe lanes.
Since then the one I have has been rock solid. I just gifted it to my kiddo b/c it's overkill for my current role. I will miss all of the jokes about my space heater though. Nothing will unify a group of 30 of my colleagues more than jeering me as I draw that beat up 17" coffee table from my backpack like some kind of nerd excalibur.
(Also more to the point I've experienced OP's issue a few times with it over the years.)