For the car meets, make sure you actually drive into all 6 of them while in free roam. For some reason it doesn’t count the ones it spawns you at when loading up a new game session.
As for skills, you still accumulate XP for skill chains after you’ve unlocked all of the perks. It’s not a huge amount, but it does help. I remember having a high skill chain of over 400K after the multipliers and being awarded over 10,000 XP for it.
For the Car Meets you have to do something at the meet. Either enter a showdown or just change your car then leave. As a note: I think I had to switch to a different car at every meet (if I didn’t do a showdown)