You found your bottleneck. The specs say:
With x4 PCI Express® 2.0 bandwidth, M.2 supports up to 20Gbps data-transfer speeds.
At 1700mb / s you pretty much maxed out the the PCIE 2.0 x4 lanes. (I suppose the missing 300mb / s are used by data framing overhead* (headers etc.).)
You might as well have bought yourself a lower spec’ed NVMe SSD. Ah well, might you ever upgrade to a new MB then your SSD is up to it.
In the specs I do see a:
1 x PCIe 3.0/2.0 x16 (x16 mode)
If it’s not in use (by a discrete video card), you might add a 4 lanes M.2 to PCIe converter card to put your NVMe drive in order to enjoy its full speed.
(This one works good and costs little.)
To determine the actual amount of data that can be transferred, the encoding technique must be understood. PCIe Gen 3.0 and PCIe Gen 4.0 use a 128b/130b encoding technique. Older generations such as PCIe Gen 2.0 use 8b/10b encoding. This encoding technique (PCie 4.0) transforms 128-bit data into 130-bit line code. This allows for reasonable clock recovery (which is the process of extracting timing information from a data stream) and ensures alignment of the datastream. Source
For PCIe 2.0:
5GT/s * (8b/10b) = 4.000Gbps
4.000 Gbps / 8 b/Byte = 500 GB/s (per lane)
Overhead: ([10b-8b]/10b)*100 = 20%