Is it safe to disable all permissions of system apps using shizuku and permission manager x ?
do restricing permission to system apps cause bootloop ? Or any crash in the system that eventually lead to a factory reset to make my device usable again ?
Is there any guides available for this ?
It is per package basis. It is generally advisable to not touch low level packages.
Low level means ? In canta like the unsafe and expert list of s/m apps ?
Low level means more important and critical things. For example, decreasing brightness on your phone is a high level thing, installing app is slightly lower, developer mode is lower, rooting your phone is even lower.