Just to update you (in case it sways you one way or the other):
I contacted Amazon to try to sort why I couldn't get 4K HDR, it basically came down to the content just being very difficult to find, but I discovered something else.
While I had read that Amazon was supporting both Dolby Vision and HDR10+, it appears that they may have dropped Dolby Vision. None of the titles I tried came up as Dolby Vision (just regular HDR), and the technical support person I chatted with said Dolby Vision wasn't available.
Take it with a grain of salt, as I have only tried a couple of titles on Amazon and the technical support person I chatted with probably isn't very high up in the chain.
Netflix is still strictly Dolby Vision for now.