The write speed being faster than read speed is strange and not normal.
Have you run the SSD Optimizer on your 520's in the Toolbox?
What does the Toolbox show your 520's health as? Even with 4TB writes, another SMART value looks like the health would be 100%. Did you try running at least the Quick Diagnostic Scan on your 520's?
Information about your PC is needed, what mother board do you have? What SATA mode are you using and set to in the BIOS? I have no idea what SATA ports your 520 is connected to, so can't comment on that.
You should try running AS SSD benchmark on one of your 520's, and post a screenshot of the results. It's a free program, just Google it and download.
The unsafe shutdown count seems to have a bug with all 520's, normal restarts of the PC are usually counted as an unsafe shutdown. That is nothing to worry about.
Your Uncorrectable Error Count seems high, but usually clears up after a reboot or two, have you done that lately?