Taking things in a different direction, why do you need 4K60?
I ask because so far the only media I've seen in that format is test media, not real media, and the GUI will perform (and look) better at 1080p than 4K because processing a 1080p GUI takes less effort and the TV will do a better job of upscaling Kodi 1080p output than Kodi will do upscaling a 720p or 1080p skin to 4K. Since real 4K media is 99.99% in [email protected], there's no need for the config.txt enable 4K60 (better for Pi thermals and power).
Doesn't explain why you can't achieve 4K60, but if you don't need to.. ?