If you are getting speckles, you might try setting the Apple TV to 4K SDR 60. Yeah, I know, but then go into video settings and click Match Content. Then, match Dynamic Range and Frame Rate. Trying to put out HDR @60hz is not advisable when you are already short on bandwidth and making a long run. Setting the menus to HDR looks worse than setting it to 4K SDR 4:4:4 because the menus are SDR and the Apple TV is just applying FAKEDR. Even with it set at 4K SDR 4:4:4, HDR 4:2:2 @24fps will be displayed when watching HDR movies after Match Dynamic Range and Frame Rate are selected. You will notice a flicker when returning to SDR from HDR @24fps. This is normal for the setting. Anyway, just something to try.