what most people don't know about ssd's is that TLC and later ssd's achieve their sequential R/W speeds by having a "buffer of emulated SLC to write to, this buffer is then xfered to the TLC during periods of drive unuse
if you exceed this buffer, the ssd xfer speeds can plummet to mech hard drive speeds which is not what you want during video work!!
Older MLC based drives (or newer ones using MCL nand) will not have this issue the samsung 850 pro is a MLC nand based ssd and due to this maintains the ssd write speed no matter how big the file is
samsung drives are usually TLC for the EVO line and MLC for the "pro" suffix models
so you're 970 pro is a MLC nand based ssd and will work fine for video work using the m.2 pci-e adapter