Welcome!
I think something is seriously out of order with your SSD. Your scores:
Performance:
System Load: 1.96 (1 min ago) 2.66 (5 min ago) 3.46 (15 min ago)
Nominal I/O speed: 1.66 MB/s
File system: 126.05 seconds (timed out)
Write speed: 196 MB/s
Read speed: 516 MB/s
From my 2012 13-inch MBP also retrofitted with a 6GB/sec SSD, but from OWC:
Performance:
System Load: 1.61 (1 min ago) 1.60 (5 min ago) 2.94 (15 min ago)
Nominal I/O speed: 0.15 MB/s
File system: 28.20 seconds
Write speed: 487 MB/s
Read speed: 482 MB/s
1) Your Write speed is far below nominal
2) File System check went over two minutes and timed out
The File System time and your description suggests intermittent stalls, and that could be an easy fix.
When you installed the SSD, did you replace its ribbon cable at the same time? If not, that could be the problem.
The only weak spot in the 2012 non-Retina MBPs is the drive cable. It is super-thin, bascially a flexible printed circuit board. The insulation is also thin and, because of a circuitous routing over some sharp metal edges on the chassis, the insulation can wear with the vibration of normal use and transport after 3-5 years. Symptoms of a worn cable can mimic those of a failing hard drive.
The good news about the cable:
For what you have posted (we greatly appreciate your including very good details) I would replace the cable before doing anything else.
If that does not improve things, consider a different SSD. Samsung was once the gold standard for SSDs but that rep is getting tarnished. Best results seem to come with Crucial and OWC solid-state drives.