iMac Pro external display?

A friend wants to add an external display to the iMac pro but finds the LG ultrafine 5K cost prohibitive and was also put off by earlier reports of quality issues when it was introduced.
There are various dongles to connect HDMI displays, but some are limited to 30Hz, also not acceptable.
So, does anyone have recommendations for either a USB-C 4k display or some way of connecting an HDMI display to an iMac Pro USB-C port.

I’m running a pair of Dell P2415Q displays (recommended by the Wirecutter and Katie Floyd) as external displays on my iMac. They have DisplayPort inputs, which support 60hz refresh rates using a USB-C to DisplayPort cable.