The 2020 Frame isn't the brightest HDR TV, but I can watch any HDR content on my Blu-ray player or the built-in apps in the daytime.
The initial v1060 firmware that was on my 65" was dark, but that seemed to be fixed in the v1113 release.
Make sure you are using the most current firmware (1301.5). You can download it here if your TV isn't installing it automatically:
For extra brightness, set "Contrast Enhancer" to High. Recommended setings can be found here:
Thanks for the links! Contrast Enhancher helped a bit. Would still prefer an option to disable HDR and/or force 1080p as all content on my Apple TV 1080p looks fine but I would prefer to stick with the TV apps vs using an external source.