Individual plot completion times are not a useful way to measure what the system is capable of. How many plots are you achieving per day? (count the file modified timestamps for a 24 hour period).
8 parallel plots on a 2TB 970 EVO Plus seems too high for these consumer TLC drives. Try 5-6. Check the iowait to determine the optimum number before it starts to struggle (see link below).
Lack of PCIe bandwidth is not the problem in your case but the 980 PROs may perform better depending on their sustained write performance (check the reviews).
You didn’t mention the OS. Linux performs better if you aren’t already using it. Proper software configuration plays a big part too.
Read this thread for lots of tips: