Further test results with SuperSU full unroot and Manually delete method. I want to remove root access from my boxes.
X96 Mini Amlogic S905W box -- SuperSU full unroot works, not so sure what happened previously
R69 Rockchip RK3229 box -- SuperSu full unroot works
H96 Pro+ Amlogic S912 box-- SuperSu full unroot works
Tanix TX5 Max Amlogic S905X2 box -- SuperSu full unroot works.
Tania TX6 Allwinner H6 box -- SuperSu full unroot does not work, manual delete files using ES File Explorer app's root explorer function works.
H96 Max H2 Rockchip RK3328 box -- I am able to recover from the RK3328 H96 Max H2 box now. So I recovered it to the same stock firmware first, backed up the superuser.apk first as the lesson learned from previous experiment. SuperSU full unroot does not work. Manually delete the files and remove the system superuser app does not work either since /sbin/su comes back after reboot (/system/xbin/su is gone). But this time I have backed up superuser.apk so I can get it back installed to manage root access.
Enough of experiments. I will leave the H96 Max H2 box as it is now -- rooted but with superuser app managing the root access.
However this full unroot still does not work for apps like SingTel TV Go, SingTel Cast and probably Astro Go official app.
For SingTel TV Go, SingTel Cast, go for Mi Box S.
For Astro Go, based on Malaysia forum, go for Minix NEO U1 or Minix NEO U9-H. And there are methods to get it work for Tanix TX3 Mini (root switch) and TX5 Max (unroot first).