Where are you getting this estimated speed from? This looks more like the manufacturer specs for a single device not the spec for a ZFS pool. 14GB for a mirror to read looks like a theoretical maximum of both drives (7GB + 7GB), which is not how it actually works. We all wish it did but it doesn’t.
As for the Crucial P5 drives, the maximum speed for write is 1800MB/sec once the onboard cache is full. That is a huge distinction. Cache are great for small pieces of data but 200GB (making an assumption here) fills up quick and the NVMe chips still write at 1800MB/sec.
And while I may be wrong on some of the specifications for your drives, this is still the way things work in the real world.
Have you read this white paper from the company, it helps, also read the thread as there is some good information there as well?
And one more thing to note with respect to write speeds, TrueNAS has TRIM turned off by default as far as I’m aware, you may manually run trim and the run the write test again and the values may improve.