Re fixing the Primes system - it's pretty obvious that 5e had some C&C influence with the Proficiency Bonus, and I'd use that instead of the C&C Primes bonus/TN reduction: +2 at 1-4, +3 at 5-8, +4 at 9-12, +5 at 13-16 and +6 at 17-20. With a base TN of 15 rather than 12/18.
This reduces the value of Human PCs getting 3 Primes. I'd suggest give them +1 STR +1 CON as well, or even two floating +1s, assign as desired.