If you buy an off-the-shelf 5K monitor, like LG’s UltraFine displays, they’re very different from the design of the iMac. The frame is black and made of plastic, and aesthetically, these models aren’t really appealing either, even though the image display is really good.
In the old days, when Apple monitor design was still important, you could get a Thunderbolt display next to your iMac. These were slightly different, but at least the design language was the same and both devices were made of aluminium.
So there’s nothing left to do but follow in the footsteps of Leo (see video) and build yourself an iMac-designed monitor. All you need is an old iMac case (from eBay for example) and
In principle, everything is really modular: The 5K panel is operated with the control board, which in turn receives the input signal from the Thunderbolt/HDMI adapter. Everything is neatly built into the case (make sure that the boards don’t touch the aluminium back and don’t play around with the 230 volts if you don’t know what to do!) and you have a monitor that finally looks good next to your iMac.