Try this Kodi-GBM test build of LibreELEC. It does not use the Xorg stuff, so 60Hz should be usable.
This is a step in the right direction: it does permit 60hz resolutions for 2840x2160 and 4096x2160 for the interface, and I had no playback problems for files that were less than 4K resolution. Playback of 4K content caused strange screen problems, though the projector recognized the signals.
Thanks for the help. I would not have suspected that there would be an issue with the Intel Graphics driver ...