You don’t need to read the whole 600 pages before session 1. Read:
The defining feature of Kingmaker is the integration of subsystems that usually act as distractions in other campaigns. In the PDF, these mechanics are critical: pathfinder 2e kingmaker adventure path pdf
If your group wants a campaign where their choices literally change the map and they can play the same characters from level 1 to 20, Kingmaker is the gold standard You don’t need to read the whole 600