Contrary to what I was initially thinking, the final round isn't actually the round that you get to Kingmaker, it's the round before. It takes 90 stars to reach the final round. Once you get to about 40 or so, there will be a few skill challenges (drift zones, speed zones, danger signs and speed traps) unlocked every round that actually make progressing through the rounds fairly quick. I only had to do a handful of races after I reached 50 stars to get to 90. I used a fully upgraded STi rally car to do all the challenges aside from the bucket list challenges of course.
edit: If you obtain the Terradyne Gurkha via Forzathon and tune it to A800, it makes all the races very easy. Thanks to gresnik for this tip!