I agree with everything else, but have to point out that the first digit in J-1X (not very sure of J-2X) is not meant for generation. PLA aircraft so far goes by simple number increments.
1. There was Nanchang J-12 which was a development after J-6, lighter and simpler than J-7. It was never introduced because J-7 was better.
View attachment 78887
2. J-13, both SAC and CAC proposed their designs, but J-13 development was stopped in 1981 to save money. J-13's design requirement was somewhere between J-8 and later J-10.
This is why there are many holes in the J series. I think we can ccount every number up to 17.