Found the same in terms of 4k blurays. I have an X1X anyway so the player front is sorted unless I feel an upgrade to a proper one but still stings paying that for some films. I went down the Apple TV 4k route for that for 4k films. A lot of the new films (seems to be most things except disney films) are in 4k dolby vision and dolby atmos is soon to come in the new tvOS release in september. Helps that the maximum a new film is, is about £14, with many older being down to £4-8 and full whack for a 4k bluray for disney releases lol. Prime is just a bit of a mess for content discovery. You got to keep scrolling down for ultra hd content, or maybe searching ultrahd or ultra hd may work.