I'm curious if you had done the probability calculations for the chance of a mishap.
https://anydice.com/program/2fe04. In this link, the value of 0 is the chance of completely separate die values.You can change the 5d6 to the different rolls to get the below values.
P1: 0%
P2: 16.67%
P3: 44.44%
P4: 77.22%
P5: 90.74%
So Power 5 is practically guaranteed to mishap with power 4 being ~3/4 chance. Was the intent that most players would never cast a power 4/5 spell as they don't want to risk the mishaps?