This is a heads up to those stuck at 96% and are looking at solutions that require managing offline saves etc.
Before you do any of that, if you are stuck at 96%, make sure you aren't making a silly mistake.
The achievement is for all perks, excluding temporary perks. That's the top row on each skill right? Wrong.
The top row on the two left trees, Festival Boss and Skills are temporary. You can tell as below there is a 0 (or however many times you have unlocked it). The top row on the right tree, instant skills, do not have this. They aren't temporary. The top row on the instant skills tree is required.
And if you do the math.....all skills except the top rows on all three trees equals. You guessed it, 96%.
So make sure the top row on the instant skills tree is unlocked, as well as all three skills at the very top which you give you the three bonus cars (one from each tree), this will give you 100% without having to delete games. And it's far quicker to earn the skill points with all ugorades than running through the start of the game again.
Hope this helps, as I thought mine was bugged until I worked this out.