I’m using CoreELEC on the Fire TV 2nd gen Cube (s922x) with Dolby Vision working. The Cube uses a 64bit Dolby Vision module which is a little different than the AM6 & U22X-J that use a 32bit module. Perhaps the AM6 & U22X-J could also make use of the Cube’s DV module.
I made a separate post with a little more details as to not sidetrack the discussion here.