> the old Target Display mode which was unfortunately dropped from the iMac 5K

FWIW: It was dropped from that model because (at the time) there was no way to drive a monitor that large over DisplayPort. DisplayPort 1.3 would have supported it, but that was only announced a month or so before the iMac 5K came out -- the hardware was finalized long before that.

It is disappointing that Apple hasn't added Target Display Mode back to newer models, though.