Okay, I have tried it with the StarTech dock. I can now confirm that MST (5K over dual DP connection) is NOT supported with this monitor. Best I could get was a side-by-side picture from two inputs. That's what the supplier told me would happen.
I could probably try it with just a TB3-2DP dongle, but it's rather expensive just to try something that most likely wouldn't work...
A couple of words on the StarTech Triple 4K USB-C Laptop Docking Station product id DK30CH2DPPD — it can be a good solution to some, but definitely not for me, since its DP ports are running through DisplayLink. There's a long list of associated problems (no clamshell mode, high CPU load, bad Retina handling etc.) There is something good about it though: it does charge the laptop, and it gives you a USB hub, and probably Ethernet (didn't check whether Ethernet worked without DisplayLink drivers).
tl;dr:
Two expensive options are left: trying a eGPU, or actually buying an LG Ultrafine 5K display. Haven't decided yet.