I haven't looked specifically for what you are after but my understanding is that you are right. There are no docks that switch between DisplayPort pass through over Thunderbolt 3 and DisplayLink over USB2/3/3.1/whatever.

I'm not sure a Thunderbolt 3 hub can plug into a conventional USB2/3.x port to start with and that's ignoring any of the video issues.

I reckon most people wouldn't want to pay for both solutions in one dock.

Note that DisplayLink can go higher than 1920x1080 (I think the old limit was 2048x1152 actually). I see there's even a 4k 60Hz capable one now:
http://support.displaylink.com/knowledgebase/articles/525038-displaylink-display-resolution-support

So maybe just use USB 3.1 (hopefully 3.1 Gen 2 so you get 10 Gbps instead of 5) with a DisplayLink dock or a couple of DisplayLink adapters?

I haven't seen any hubs with that latest 4k 60Hz chipset yet but haven't looked at them for a few months. Expect it to take a while to filter through based on previous experiences.