The OTA problem has been resolved. Look like Magisk Manager messed up the bootloader and that is why OTA updates failed.
Anyway, I use Phoenix Card to burn an official ROM from Tanix website (email address required), then I am able to carry out the OTA.
The Phoenix Suite and Phoenix USB Pro failed this time due to USB driver recognition problem for my Windows 10 laptop.
Ha-ha, now I learned the recovery for both Amlogic CPU based box and Allwinner CPU based box.
Hmm, do not upgrade to the latest 20190508 version. Now I got no sound problem. One user in Facebook reported the same problem.
Now need to troubleshoot the sound problem and may need to flash the firmware again...
All in all, this Tanix TX6 is really problematic.
Opened the box and I feel the thermal design is not correctly done which causes the overheating problem. They have a small heat sink on top of the CPU and then connect to a bigger heat sink on the bottom through a small piece of thermal grease cube.