Alright. I didn't get the adapter yet and i don't mind getting the StarTech one or anything else. Maybe you could run AGDCDiagnose, and then we'll know if it supports HBR2 ? If so, the dock doesn't matter, right?
The StarTech has a max link rate of HBR. One strange thing is that it wouldn't work with two different USB-C to DisplayPort adapters I have (white Moshi and Cable Matters DisplayPort 1.4) connected to a Thunderbolt 3 port of my Mac mini 2018, or a OWC Thunderbolt 3 Dock, or a HP Thunderbolt Dock G2. It would detect and get the DisplayPort info but it wouldn't setup the display - it remained black. The Mini DisplayPort port of the OWC Thunderbolt 3 Dock also didn't work.
It did work after I connected it to a DisplayPort port of a HP Thunderbolt Dock G2 (which has a built in DisplayPort 1.4 MST hub so the connection is HBR2 to the Mac mini). The USB-C adapters worked when connected to the USB-C port (with DisplayPort alt mode) port of the HP Thunderbolt Dock G2 (which is also connected to the built in DisplayPort 1.4 MST hub). Of course, the Thunderbolt port of the dock is not connected to the internal MST hub.
All my connections were four lanes because I don't have a two lane source to test. I don't have any USB-C hubs with non-MST DisplayPort output.
Does this mean my Mac mini 2018 won't do HBR? That would be weird.
I tried connecting the Apple 30" Cinema Display to the HDMI 2.0 port of the Mac mini 2018 (must add 1280x800 timing with SwitchResX because that is the max for single link connection for this display). The DisplayPort rate is HBR2 for the DisplayPort to HDMI 2.0 converter even for low res timings.
I used low res timings on my 4K display (640x480 and 2560x1440) but the connection link rate still remained at HBR2 from the Thunderbolt 3 port.
I guess I need a real DisplayPort 1440p display to test HBR link rate. Oh wait - my 4K display has a DisplayPort 1.1 mode. In that case, my 4K display does switch to HBR link rate (for 4K30, and 1440p60).
Ok, so HBR link rate can work from my Mac mini 2018 Thunderbolt 3 ports. I just don't understand why the StarTech adapter is not working directly. Maybe you should go with the Apple adapter?