how to block an OS app popup?
I managed to minimize the problem by using the "do not disturb" feature.
By "minimize" I mean that if I am using the phone the popup will still show up, otherwise it wont mess it's battery (neither show up).
I can allow all exceptions least "reminders" apparently.
Obs.: "do not disturb" is not 100%, I think about 10% of the times it will still allow that abusive popup to keep the screen ON for many hours!

The simToolkit app package "com.android.stk" (which I found out using Brevent), is called "Serviços TIM" and seems to be a mirroed/dup (???) package from the default simToolkit.  

My next step (as soon that popup annoys me enough) is to try to uninstall that package by using this tip by @Izzy, thx!
How to remove the SIM Toolkit from my phone? (Without root or flashing any ROM)

adb uninstall failed
Uninstall via adb without root
How to disable Google Apps in Mi Phones
Did it finally:

adb shell pm list packages |grep stk

adb shell pm path com.android.stk
adb pull /system/app/Stk/Stk.apk # to backup it just in case

adb shell pm path com.android.stk2
adb pull /system/app/Stk2/Stk2.apk # ...

adb shell pm uninstall -k --user 0 com.android.stk
adb shell pm uninstall -k --user 0 com.android.stk2