How about making the Iso-8 reward for the completed “Prologue” missions random between 20 and 40 Iso-8. It’s a relatively small change. Similar to the 70 or 140 Iso-8 reward when playing in tournaments.
Hmm… too not increase the average gain, I suggest changing it to 10 or 30 instead. That’d make it similar to tourney rewards without it being too good.
Might as well do 0 to 40 
I still think it should be based on difficulty.
Trivial: 10
Easy: 20
Hard: 40
Impossible: 80
Trust me, you do not want this.
all of my missions are trivial now lol