I've owned an LG 4K HDR TV for a few weeks now and initially I found getting the HDR picture to look its best quite baffling. The solution is surprisingly simple though. After lots of fiddling with all manner of controls I've discovered that avoid the picture being too dark in HDR UHD disks or HDR Xbox One S games is merely a matter of adjust the TV brightless level. I wrote the following response on a forum but I thought it was worth sharing here.
Update: Since I first posted this, LG has released a firmware update to add a low-latency HDR mode for gaming. This new mode appears alongside the other three HDR modes.