Finally, I obtained a smooth playback, no interruptions. 1090p and 720p/24 will play 4:4:4:-12bit. 720p/25fps will play 720p/25, the machine cannot upscale to 1080p, but it plays smooth, also at 4:4:4:-12bit. 4kHDR will play at 4:4:4 - 8bit, the max my HDR tv panel will take. I am pleased.
I use AFR script V26 from #Pcristi, from here https://www.mediafire.com/?qu0b14itqs1d4 
For whomever interested, this is how I modified the settings file, afr.cfg from sdcard directory.
3840X2160:444, 12 
3840X2160@59.94:3840X2160@60, 444, 12 
1080:444, 12 
1080@59.94:1080@60, 444, 12 
1080i:1080, 444, 12 
1080i@50:444, 12
 1080i@59.94:1080i@60, 444, 12 
1080i@60:444, 12
 4096X2160:444, 12 
480:1080, 444, 12 
480@60:444, 12
576:444, 12 
720:1080, 444, 12 
720@25:444, 12
 720@30:444, 12 
720@50:444, 12 
720@59.94:720@60, 444, 12 
720@60:444, 12 
Disabled:0 
MainMenu:1080@60, 422, 8
PauseMainMenu:1