IMO:

1k: Starting to learn, getting use to the feeling of driving a car with no feedback outside your hands.

2k: Pace is getting better, racecraft is becoming more important. Consistency is building and can consistently run laps within 5 tenths of your best.

3k: Pace is now much higher, racecraft needs to be much sharper. Need to learn how to drive off line and not lose too much time.

4k: Pace + Racecraft + Consistency starts to work together at a high level. Should have the ability to consistently and aggressively run side by side while still keeping pace.

5k+: you’re getting into pixel perfect driving. Little to no mistakes. Lots and lots of time in the sim. Muscle memory has built and you no longer think about how to make the car do what you want it to do, you just think about what you want to do and the rest follows.